Zero-kinetic-energy pulsed-field ionization spectroscopy of the a1Δ state of SH+ (SD+)

Article abstract of DOI:10.1063/1.470848

The results of a zero-kinetic-energy pulsed-field ionization study on the a ¹Δ (v⁺ = 0) excited ionic state of SH⁺ (SD⁺) obtained via two-photon excitation of the [a ¹Δ]3dπ ²Φ (v′ = 0) Rydb

Full text of DOI:10.1063/1.470848

Zerokineticenergy pulsedfield ionization spectroscopy of the a 1Δ state of SH+ (SD+)
J. B. Milan, W. J. Buma, and C. A. de Lange
Citation: The Journal of Chemical Physics 104, 521 (1996); doi: 10.1063/1.470848
View online: http://dx.doi.org/10.1063/1.470848
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Zero-kinetic-energy pulsed-field ionization spectroscopy
1
of the a ⌬ state of SH؉ (SD؉)
J. B. Milan, W. J. Buma, and C. A. de Lange
Laboratory for Physical Chemistry, University of Amsterdam, Nieuwe Achtergracht 127,
1018 WS Amsterdam, The Netherlands
͑Received 28 August 1995; accepted 3 October 1995͒
1
The results of a zero-kinetic-energy pulsed-field ionization study on the a ⌬ ( ϩϭ0) excited ionic
v
state of SHϩ ͑SDϩ͒ obtained via two-photon excitation of the [a ⌬]3d␲ 2⌽ ( Јϭ0) Rydberg
1
v
state and subsequent one-photon pulsed-field ionization are reported. Accurate ionization energies as
well as rotational constants are obtained. A detailed comparison between the rotational branching
ratios resulting from the pulsed-field ionization process and those of a direct ionization process is
made. The results elucidate the dynamics of the high-n Rydberg states involved in pulsed-field
ionization of SH ͑SD͒. © 1996 American Institute of Physics. ͓S0021-9606͑96͒01202-8͔
I. INTRODUCTION
mosphere from natural and anthropogenic sources. More-
over, as the result of the relatively high cosmic abundance of
sulphur, the molecule is also of interest from an astrophysical
point of view. Nevertheless, detailed information on the
spectroscopic and photochemical properties of the SH ͑SD͒
radical and, especially, its molecular ion is relatively
scarce.6,16–26
Zero-kinetic-energy1 ͑ZEKE͒ photoelectron spectros-
copy, and especially its more widely used pulsed-field modi-
fication, zero-kinetic-energy pulsed-field ionization2 ͑ZEKE-
PFI͒ photoelectron spectroscopy, have in recent years been
developed into an extremely powerful tool to perform high-
resolution spectroscopy on molecular ions. The majority of
the studies reported so far have concentrated on high-
resolution spectroscopy of molecular ions in their electronic
ground state,1–9 although recently a few ZEKE-PFI studies
have demonstrated that this kind of spectroscopy can be ap-
plied equally successfully to the study of electronically ex-
cited states of the molecular ion.10–15 In these latter studies a
single VUV photon was employed to excite a neutral mol-
ecule in its electronic ground state to high-n Rydberg states
converging upon electronically excited ionic states, which
were subsequently ionized by pulsed-field ionization into an
electronically excited state of the molecular ion and a free
electron.
In this work we have extended this approach to study
electronically excited ionic states with ZEKE-PFI spectros-
copy by employing a low-lying Rydberg state with an ex-
cited ionic core as the ‘‘initial’’ state for one-photon pulsed-
field ionization. This low-lying Rydberg state is populated by
two-photon resonant excitation from the ground state en-
abling selective excitation of just one particular vibrational
and rotational level. It is clear that this combination of mul-
tiphoton excitation and one-photon pulsed-field ionization
carries some distinct advantages over VUV ZEKE-PFI stud-
ies, which use the electronic ground state as the ‘‘initial’’
state for pulsed-field ionization. First of all, it allows the
investigation of the spectroscopic properties of excited ionic
states without the need to resort to VUV excitation sources.
Secondly, the pulsed-field ionization process can be studied
starting from a single vibrational and rotational level of the
intermediate Rydberg state, thereby reducing considerably
the number of resonances in the ZEKE-PFI spectra.
The
SH
͑SD͒
radical
has
a
2
2
2
4
2
2
3
2
͑1␴͒ ͑2␴͒ ͑3␴͒ ͑1␲͒ ͑4␴͒ ͑5␴͒ ͑2␲͒
(X ⌸)
electron
ground state configuration. Spin–orbit interaction ͓A0Љ
ϭ Ϫ376.835 cmϪ1͑SH͒ ͑Ref. 23͒; Ϫ376.75 cmϪ1͑SD͒ ͑Ref.
2
16͔͒ leads to an inverted splitting with the ⌸3/2 state below
the 2⌸1/2 state. Removal of an electron from the 2␲ molecu-
lar orbital, which is essentially a nonbonding 3px atomic
orbital centered on the sulphur atom, gives rise to three dif-
3
ferent ionic states: the X ⌺Ϫ ground ionic state, and the
1
1
3
a ⌬ and b ⌺ϩ excited ionic states. The higher lying A ⌸
1
and B ⌸ excited ionic states derive from the removal of an
electron from the 5␴ molecular orbital.
Until recently the ionization energies for the aforemen-
tioned ionic states were only known with an accuracy of 0.01
eV ͑80 cmϪ1͒ from HeI VUV photoelectron spectroscopy
͑PES͒ measurements.19 A more accurate determination of the
3
ionization energy of the X ⌺Ϫ ground state ͑10.37 eV͒ was
recently achieved by the rotationally resolved nonresonant
two-photon pulsed-field ionization ͑N2P-PFI͒ studies of Hsu
et al.6 leading to a value of 84 057.5Ϯ3 cmϪ1. Similarly,
relatively little is known about the vibrational and rotational
constants of the ionic states. Accurate values have only been
3
3
determined for the X ⌺Ϫ and A ⌸ ionic states by absorp-
tion and emission studies of the A ⌸ϪX ⌺Ϫ band
3
3
system.18,20,21
1
Here, we will be concerned with the study of the a ⌬
excited ionic state of the SHϩ ͑SDϩ͒ ion. To this purpose we
will employ the low-lying [a ⌬]3d␲ 2⌽ ( Јϭ0) Rydberg
1
v
state of SH ͑SD͒, excited by two-photon excitation from the
ground state, as the ‘‘initial’’ state in the approach described
above. Recently, we have applied ͑2ϩ1͒ resonance enhanced
multiphoton ionization photoelectron spectroscopy ͑REMPI-
PES͒ to characterize this hitherto unobserved Rydberg
The subject of the present study is the mercapto radical
͑SH͒, which plays an important role in the ultraviolet photo-
chemistry of sulphur-containing species released into the at-
J. Chem. Phys. 104 (2), 8 January 1996
0021-9606/96/104(2)/521/7/$6.00
© 1996 American Institute of Physics
521
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522
Milan, Buma, and de Lange: Spectroscopy of SHϩ
state.27 Analysis of its rotationally resolved two-photon exci-
tation spectrum unambiguously showed it to be of ⌽ sym-
metry and allowed for an accurate determination of its spec-
troscopic parameters. Nonrotationally resolved photoelectron
spectra for ionization via low rotational levels of the 2⌽ state
could be reached between distinguishing prompt and PFI
electrons, while still retaining enough PFI electrons to obtain
ZEKE-PFI spectra with a reasonable signal-to-noise ratio.
In a second type of experiment, electrons were moni-
tored that derived from various other ionization channels,
i.e., near-zero-kinetic-energy electrons resulting from direct
2
revealed that the ionic core associated with this particular
1
1
2⌽ state is the a ⌬ ( ϩϭ0) excited state of the ion, and
ionization and/or rotational autoionization into the a ⌬
v
that this ionic core is not, within our signal-to-noise ratio,
ionic continua, as well as electrons with a kinetic energy of
contaminated with contributions from other ionic states.
ϳ1.2 eV resulting from electrostatic autoionization and/or
1
3
Using this [a ⌬]3d␲ 2⌽ ( Јϭ0) state as the ‘‘initial’’
direct ionization into the ground X ⌺Ϫ ionic continua.
v
state in our ZEKE-PFI experiments, we have been able to
obtain accurate rotational thresholds for the a ⌬ excited
These experiments were performed under considerably re-
duced power of the probe laser to avoid saturation of the
1
state of the SHϩ ͑SDϩ͒ ion, from which its ionization energy
and rotational parameters have been derived. Moreover,
comparison of the rotational branching ratios obtained from
ZEKE-PFI measurements with those of REMPI-PES studies
and concurrent theoretical calculations26–28 have allowed for
a fundamental insight into the dynamical properties of the
high-n Rydberg states involved in our ZEKE-PFI studies. It
will be shown that these states are relatively insensitive to
autoionization processes, which have been investigated sepa-
rately in non-ZEKE experiments.
one-photon transition from the [a ⌬]3d␲ 2⌽ Rydberg
1
state to the high-n Rydberg states converging upon the rota-
1
tional limits of the excited a ⌬ ionic state, and with only a
small dc electric field ͑1 V/cm͒ which merely served to ex-
tract the near-zero-kinetic-energy electrons. H2S ͑99.6%,
Messer Griesheim͒ and D2S ͑98%D, Campro Scientific͒
were used without further purification. Each was introduced
into the spectrometer, in turn, as a continuous effusive leak.
III. RESULTS
A series of ZEKE-PFI spectra obtained by two-photon
II. EXPERIMENT
1
excitation of various rotational levels of the [a ⌬]3d␲ 2⌽
A detailed description of our setup for laser photoelec-
tron spectroscopy has been given elsewhere.29 In the present
experiments counterpropagating beams from two dye lasers,
both pumped by the same excimer laser, were employed.
One frequency-doubled dye laser, operating on Coumarin
500, provided photons for the photolysis of H2S ͑D2S͒ via
excitation to the first dissociative absorption band which
spans the wavelength range 180–270 nm, leading to SH
( Јϭ0) state via the S (3/2) to S (13/2) rotational transi-
v
1 1
1
tions, and probing the excited a ⌬ rotational ionization
thresholds by one-photon pulsed-field ionization is shown in
Figs. 1͑a͒–1͑f͒. A similar series could not be obtained for the
SD radical because of a considerably lower signal-to-noise
ratio in these experiments: here only excitation of the
1
[a ⌬]3d␲ 2⌽ ( Јϭ0) state via the S (3/2) rotational tran-
v
1
sition permitted ZEKE-PFI experiments. This spectrum is
depicted in Fig. 2.
2
͑SD͒ radicals in their X ⌸ ground state.24 The same photons
were subsequently used for the two-photon resonant excita-
ZEKE-PFI spectra obtained by pumping the intermediate
rotational levels NЈϭ3 and 4 via the Q1 and R2 rotational
branches ͑not shown here͒ are barely different from those
obtained by pumping via the corresponding S1 rotational
branches ͓see Figs. 1͑a͒ and 1͑b͔͒. Similarly, the ZEKE-PFI
spectrum obtained by pumping the NЈϭ6 rotational level via
the Q1(13/2) rotational transition ͑not shown here͒ hardly
differs from that obtained via S1(9/2) ͓see Fig. 1͑d͔͒. Due to
1
tion of SH ͑SD͒ to the [a ⌬]3d␲ 2⌽ ( Јϭ0) Rydberg
v
state. The second dye laser, operating on DCM, was used to
1
scan the rotational ionization thresholds of the excited a ⌬
(
ϩϭ0) ionic state.
v
Resonances of xenon and atomic sulphur ͓which is also
produced by the photolysis of H2S ͑D2S͒ and SH ͑SD͔͒ in the
two-photon energy range of 79 500–89 800 cmϪ1 were used
to calibrate the pump laser.30,31 The probe laser was cali-
brated using optogalvanic lines of Ne excited in a hollow-
cathode discharge.32
t
the proximity of the S1(9/2) and S21(3/2), and of the
t
S1(13/2) and S21(5/2) rotational transitions, Figs. 1͑d͒ and
1͑f͒ show additional lines corresponding to ionization from
NЈϭ4 and NЈϭ5, respectively. These lines are marked with
asterisks in Figs. 1͑d͒ and 1͑f͒. The rotational branching ra-
tios observed for these transitions are almost identical to
those seen in the spectra of Figs. 1͑b͒ (NЈϭ4) and 1͑c͒
(NЈϭ5), respectively. It can therefore be concluded that
alignment effects play a relatively minor role in the present
experiments.
Field-free ionization thresholds can be extracted from
ZEKE-PFI experiments if one can correct for the Stark shift
of the ionization threshold caused by the various electric
fields present in the experiments. In our study these fields
concern the dc field of 4 V/cm, which is used to sweep out
the prompt electrons, the pulsed-field of 20 V/cm, which is
The experiments were performed using a ‘‘magnetic
bottle’’ electron spectrometer. On the pole faces of the 1 T
electromagnet a pair of grids is mounted which allow for the
application of dc and pulsed electric fields. In the present
ZEKE-PFI experiments, a fixed dc electric field of 4 V/cm
was applied to sweep out the prompt electrons. After an ap-
propriate delay of ϳ60 ns, a fast electric field pulse of 20
V/cm was used to ionize the high-n Rydberg states. Although
it is common practice to use considerably larger delay times,2
such delay times did not turn out to be possible in the present
experiments. As the result of the relatively fast decay of the
high-n Rydberg states on a time scale in the order of 40 ns,
the delay time had to be chosen such that a compromise
J. Chem. Phys., Vol. 104, No. 2, 8 January 1996
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Milan, Buma, and de Lange: Spectroscopy of SHϩ
523
two electric fields by extrapolation to zero-field conditions,33
but accounting for the latter fields is considerably more prob-
lematic since their strengths are almost impossible to deter-
mine. We have therefore adopted another approach to correct
for the combined Stark shifts resulting from all fields by
seeding the sample with xenon and performing a ZEKE-PFI
experiment on xenon via the two-photon excited 6p[1/2]0
resonance at 80 119.5 cmϪ1 ͑Ref. 30͒ under the same experi-
mental conditions as in the ZEKE-PFI experiments on SH
͑SD͒, albeit with considerably reduced laser power to avoid
ac Stark shifts. Since the ionization energy of Xeϩ(2P3/2) is
accurately known, these measurements allow us to determine
the total Stark shift as 14.6 cmϪ1. The motional Stark field is
velocity and consequently, mass dependent. Since the mass
of xenon is about 4 times larger than that of SH ͑SD͒, one
would expect that in SH ͑SD͒ the motional Stark field would
be twice as large, thereby putting into question whether we
can use the correction factor found for xenon also for the SH
͑SD͒ radical. In this respect it is worthwhile to consider the
results of a recent ZEKE-PFI study on HBr ͑Ref. 9͒ in the
‘‘magnetic bottle’’ spectrometer, in which we have investi-
gated the influence of the electric fields in more detail. These
experiments demonstrated that the combined stray and mo-
tional Stark fields amounted to about 0.1 V/cm. Neverthe-
less, if we use this number for a worst case estimate we
conclude that the error introduced by the motional Stark field
in SH ͑SD͒ is at most 2 cmϪ1. We will therefore apply the
correction of 14.6 cmϪ1 found for xenon to SH ͑SD͒, incor-
porating the uncertainty in the motional Stark field in the
final uncertainty in the ionization thresholds.
FIG. 1. ZEKE-PFI spectra of the a 1⌬ excited ionic state of SHϩ obtained
after two-photon excitation of the [a 1⌬]3d␲ 2⌽ ( ϭ0) via the ͑a͒
vЈ
S1(3/2); ͑b͒ S1(5/2); ͑c͒ S1(7/2); ͑d͒ S1(9/2)ϩtS21(3/2); ͑e͒ S1(11/2); and
͑f͒ S1(13/2)ϩtS21(5/2) rotational transitions. Individual line assignments
are indicated by the comb above the spectrum. For the transitions marked
with an asterisk, see text.
The energy scale in the spectra presented in Figs. 1
and 2 has been corrected for this Stark shift and is given
with respect to the energy of the lowest rotational level of
2
the X ⌸3/2 ground state of the neutral molecule.
used to ionize the high-n Rydberg states, the stray electric
fields inherent in the ‘‘magnetic bottle’’ spectrometer, and the
motional Stark field induced in molecules moving in the 1 T
magnetic field. One could, in principle, account for the first
Field-free
ionization
energies
of
the
transition
3
2
1
X ⌸3/2( Љϭ0, JЉϭ )→a ⌬ ( ϩϭ0, Jϩϭ2) for SHϩ
v
v
2
2
and SDϩ can now easily be extracted from Figs. 1 and 2 as
93 925Ϯ3 cmϪ1 ͑11.6453Ϯ0.0004 eV͒ and 93944Ϯ3 cmϪ1
͑11.6476Ϯ0.0004 eV͒, respectively. The accuracy of these
numbers is determined by the uncertainties in the wavelength
calibration of the two dye lasers, and the determination of the
peak positions in the ZEKE-PFI spectra and the Stark shift.
Rotational analysis of the ZEKE-PFI spectra of the SHϩ
radical ͑Fig. 1͒ has been performed using the Hund’s case ͑b͒
1
expression for the rotational energy levels of the a ⌬ state:
2
E Nϩ͒ϭB Nϩ Nϩϩ1͒ϪD Nϩ Nϩϩ1͒
,
͑
͑
͓
0
͑
͔
0
where B0 is the rotational constant and D0 the centrifugal
distortion correction. The spectral parameters obtained from
a least-squares fit of the measured energies to the expression
given above are given in Table I. Rotational analysis of the
SDϩ radical ZEKE-PFI spectrum ͑Fig. 2͒ allowed only for
the determination of the rotational constant B0 , albeit with
less accuracy ͑see Table I͒.
Figure 3 shows the results of a non-ZEKE ͑2ϩ1Ј͒
REMPI experiment, in which a number of competing
͑auto͒ionization processes have been monitored. These
spectra have been obtained by two-photon excitation
FIG. 2. ZEKE-PFI spectrum of the a 1⌬ excited ionic state of SDϩ obtained
after two-photon excitation of the [a 1⌬]3d␲ 2⌽ ( ϭ0) via the S (3/2)
vЈ
1
rotational transition. Individual line assignments are indicated by the comb
above the spectrum.
J. Chem. Phys., Vol. 104, No. 2, 8 January 1996
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524
Milan, Buma, and de Lange: Spectroscopy of SHϩ
cited ionic state of SHϩ ͑see Table I͒. Our value for the
rotational constant B0 of 9.186 cmϪ1 is very similar to that of
9.01 cmϪ1 obtained by Park et al. from ab initio
calculations.34 However, it is significantly larger than that of
8.85 cmϪ1 calculated from the estimated equilibrium
TABLE I. Spectroscopic constants ͑in cmϪ1͒ derived for the vibrationless
level of the a 1⌬ excited ionic state of SHϩ and SDϩ. The values in paren-
theses represent one standard deviation in the last digit. Constants for the
3
Ϫ
vibrationless level of the X
⌺
ground ionic state of SHϩ and SDϩ ͑Refs.
6 and 20͒ are included for comparison.
1
Ionization
thresholds
bondlength re ͑Ref. 19͒ for the a ⌬ ionic state and the ␣e
B0
D0(104)
value ͑Ref. 34͒.
1
The a ⌬ excited state of the SHϩ ͑SDϩ͒ ion is just one
3
Ϫ
Ϫ
SHϩ
SHϩ
SDϩ
SDϩ
X
a
X
a
⌺
84 057.5
93 925 Ϯ 3
9.134
9.186͑8͒
4.733
4.9
9.0͑9͒
1.29
1
⌬
⌺
⌬
of the three states derived from the electron configuration
3
2
2
2
4
2
2
͑1␴͒ ͑2␴͒ ͑3␴͒ ͑1␲͒ ͑4␴͒ ͑5␴͒ ͑2␲͒2. The potential energy
1
93 944 Ϯ 3
4.85͑8͒
curves of the X ⌺Ϫ, a ⌬, and b ⌺ϩ ionic states are ex-
pected to be quite similar. Verification for this is found in the
comparison between the well-known spectroscopic param-
3
1
1
eters of the X ⌺Ϫ ground ionic state of SHϩ and SDϩ and
3
1
of the [a ⌬]3d␲ 2⌽ ( Јϭ0) Rydberg state of SH via the
v
our constants ͑see Table I͒.
S1(11/2) rotational transition and subsequent one-photon
ionization. The top spectrum displays the ionization yield as
a function of the total ͑2ϩ1Ј͒ photon energy when only near-
zero-kinetic-energy electrons are monitored. The bottom
spectrum is obtained when electrons with a kinetic energy of
ϳ1.2 eV are selectively monitored.
The ZEKE-PFI spectra obtained in the present study
show dramatic differences between the intensities of the vari-
ous ⌬NϭNϩϪNЈ transitions. In order to put these differ-
ences and the number of observed ⌬N transitions into proper
perspective it is instructive to consider the results of our
previous ͑2ϩ1͒ REMPI-PES experiments and concurrent
theoretical calculations on the rotationally resolved ioniza-
IV. DISCUSSION
1
tion dynamics of the [a ⌬]3d␲ 2⌽ ( Јϭ0) state.26–28
v
The experiments described earlier, have led to an accu-
rate determination of the field-free ionization energy of the
First, it is important to note that the REMPI-PES
experiments27 demonstrated unambiguously that the
1
excited a ⌬ ionic state of SHϩ as 93 925Ϯ3 cmϪ1, which is
1
[a ⌬]3d␲ 2⌽ ( Јϭ0) state is a Rydberg state with a very
v
in excellent agreement with the previously obtained value of
pure ionic core: photoelectron spectra obtained for ionization
11.65Ϯ0.01 eV.19 For the excited a ⌬ ionic state of SDϩ, a
1
1
via this state only showed ionization to the a ⌬ excited
slightly higher value of 93 944Ϯ3 cmϪ1 is obtained, in line
with the expectation of a small positive isotope shift result-
ing from the decrease in vibrational frequency in going from
ionic state and, within the experimental signal-to-noise ratio,
3
no ionization to the X ⌺Ϫ ground ionic state. This core-
preserving photoionization process was thoroughly examined
in rotationally resolved one-color ͑2ϩ1͒ REMPI-PES experi-
2
1
the X ⌸ ground state of the neutral to the a ⌬ excited state
of the ion.19,34
ments, in which ionization was established via several high
1
rotational levels of the [a ⌬]3d␲ 2⌽ ( Јϭ0) Rydberg
The least-squares fit to the line positions observed in our
rotationally resolved ZEKE-PFI spectra has led to the first
determination of the rotational parameters of the a ⌬ ex-
v
state of the SH radical, in combination with the results of ab
initio calculations. Most gratifying, it was found that the
agreement between the calculated and the measured rotation-
ally resolved photoelectron spectra was excellent. From
these studies it could be concluded that the photoionization
dynamics via this state are peculiar in two respects. First,
only transitions up to ⌬NϭϮ2 were observed, whereas tran-
sitions up to Ϯ4 were expected. Secondly, strong asymme-
tries were observed in the ionic rotational branching ratios.
Conservation of angular momentum for one-photon ion-
ization in a diatomic Hund’s case ͑b͒ limit requires that the
selection rule ⌬NϭNϩϪNЈϭlϩ1,lϪ1,...,ϪlϪ1 ͑Ref. 35͒
is obeyed, where l is the orbital angular momentum associ-
ated with the outgoing electron wave. In an atomiclike pic-
ture one would expect that on removal of a 3d␲ Rydberg
electron the p and f partial waves would dominate, from
which a rotational ion distribution with transitions up to ⌬N
ϭϮ2 and ⌬NϭϮ4, respectively, would be expected. Con-
sequently, the appearance of a maximum value of ⌬N in the
rotationally resolved photoionization spectrum from a single
rotational level should indicate the highest value of l. Since
ab initio calculations showed that the f wave continua make
the dominant contribution to the total photoionization cross
section of the 3d␲ Rydberg electron, we would expect tran-
1
FIG. 3. Ionization spectrum of SH obtained for ͑2ϩ1 ͒ ionization via the
Ј
S1(11/2) rotational transition of the [a 1⌬]3d␲ 2⌽ state: ͑a͒ ionization into
the a 1⌬ excited ionic continua obtained by monitoring near-zero-kinetic-
3
Ϫ
energy electrons; ͑b͒ ionization into the X
tained by monitoring photoelectrons with a kinetic energy of ϳ1.2 eV.
⌺
ground ionic continua ob-
J. Chem. Phys., Vol. 104, No. 2, 8 January 1996
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Milan, Buma, and de Lange: Spectroscopy of SHϩ
525
sitions up to ⌬NϭϮ4. The fact that only transitions up to
⌬NϭϮ2 were observed was attributed to interference ef-
fects between the k␴, k␲, and k␦ photoelectron continua of
the lϭ3 outgoing partial wave.
The second and most striking feature was found in the
strong asymmetry in the rotational branching ratios. Al-
though minor asymmetries are not exceptional,36,37 both the
measured and calculated rotationally resolved photoelectron
spectra showed large differences in intensities between rota-
tional transitions that only differ in the sign of ⌬N. This
asymmetrical behavior was attributed to large deviations
from the classical description of rotational motion where
gaining or loosing angular momentum would be equally
probable, and clearly originated from the high value of elec-
2
tronic orbital angular momentum of the resonant ⌽ state.
FIG. 4. REMPI-PES spectrum obtained for ͑2ϩ1͒ ionization via the S1(7/2)
rotational transition of the [a 1⌬]3d␲ 2⌽ state of SH ͑Ref. 28͒. Individual
line assignments are indicated by the comb above the spectrum.
The ab initio calculations do not predict a considerable
photoelectron kinetic energy dependence in the radial dipole
matrix elements.28 We would therefore expect the present
ZEKE-PFI results and the previous REMPI-PES results to be
reasonably similar, provided that the long-lived Rydberg
states populated in our ZEKE-PFI experiments are not
heavily perturbed by Rydberg–Rydberg and/or Rydberg-
continuum interactions. Indeed, our ZEKE-PFI spectra show
the same features: most of the intensity lies in rotational
enhanced in the ZEKE-PFI spectra: the ⌬NϭϪ2 transition
has gained intensity with respect to ⌬Nϭϩ2. Similar con-
siderations for the ⌬Nϭϩ1 and ⌬NϭϪ1 transitions are
difficult because of the unresolved ⌬NϭϪ1 line in the
REMPI-PES spectrum. However, the theoretical calculations
reveal that the transition probability ⌬NϭϪ1 for photoion-
ization via the intermediate rotational level NЈϭ5 is quite
small.28 The ⌬NϭϪ1 rotational transition, clearly present in
Fig. 1͑c͒, consequently has too large an intensity. From the
comparison between the ZEKE-PFI spectra of Figs. 1͑d͒–
1͑f͒ with the measured and calculated results for direct
photoionization28 a similar agreement is concluded, despite
transitions with ͉⌬N͉р2, although a few lines with ⌬N
ϭϪ3 and ⌬NϭϪ4 can also be seen, and the asymmetries
between the ⌬Nϭϩ1 and Ϫ1 and the ⌬Nϭϩ2 and Ϫ2
rotational transitions are apparent ͑see Figs. 1 and 2͒.
It appears that our ZEKE-PFI spectra show the same
unusual features as observed in the REMPI-PES direct
photoionization measurements and in the ab initio calcula-
tions. Before rushing to conclusions, however, we have to
examine the ZEKE-PFI spectra in somewhat more detail.
The limited resolution, inherent in the REMPI-PES spectra,
prohibits the measurement of rotationally well-resolved pho-
toelectron spectra via intermediate levels with NЈϽ5. A di-
rect comparison between the ZEKE-PFI and the REMPI-PES
results is consequently difficult for the low intermediate lev-
els with NЈϭ3 and 4. However, the ab initio calculations
indicate that the observed rotational branching ratios of Figs.
1͑a͒ and 1͑b͒ are quite similar to the calculated ones.28 For
NЈϭ5, a comparison between the two experimental tech-
niques is possible and leads to the conclusion that their re-
sults show a reasonably good agreement. This is demon-
strated in Fig. 4, where the previously obtained rotationally
resolved REMPI-PES spectrum via the S1(7/2) rotational
t
the problem of overlapping S1 and S21 rotational lines,
which leads in both the REMPI-PES and ZEKE-PFI experi-
ments to signals from two instead of one intermediate NЈ
rotational level. A peculiar observation in the ZEKE-PFI
spectra of Fig. 1 is the narrowness of some ⌬NϭϪ3 and Ϫ4
rotational transitions ͑vide infra͒.
We conclude that in this particular ZEKE-PFI study, the
rotational branching ratios resulting from the pulsed-field
ionization process, in general, resemble, to a large extent,
those of the direct ionization process. The minor differences
can all be explained when one considers the influence of
rotational interactions. High-n Rydberg states, initially popu-
lated in our ZEKE-PFI experiments, may autoionize into the
various ionization continua of lower-lying rotational levels
1
of the a ⌬ excited ionic state, resulting in a decrease of the
transition of the [a ⌬]3d␲ 2⌽ Rydberg state28 is dis-
ZEKE-PFI signal. Apart from the depletion of ZEKE-PFI
states, this rotational autoionization is also responsible for
other effects. A common phenomenon in many ZEKE-PFI
studies is the enhancement of ⌬NϽ0 transitions with respect
to the corresponding ⌬NϾ0 ones. This can be explained by
adopting a model in which low-n Rydberg states, which can-
not be field ionized directly since they lie too far below their
own ionization threshold, can autoionize into the continua of
high-n Rydberg states when these decay channels become
available upon application of the pulsed field.3 Similarly,
these low-n Rydberg states can give rise to ‘‘forbidden’’ tran-
sitions in ZEKE-PFI spectra if they autoionize into forbidden
1
played, which can be compared directly to the ZEKE-PFI
spectrum shown in Fig. 1͑c͒. The ⌬NϭϪ2 transition is the
most intense in both spectra, while the ⌬Nϭ0 and ϩ1 tran-
sitions are the next prominent ones. Despite these similarities
some discrepancies remain. First, in the ZEKE-PFI spectrum
1
͓Fig. 1͑c͔͒ the lowest rotational level of the a ⌬ excited
ionic state (Nϩϭ2) is seen, which corresponds to a ⌬N
ϭϪ3 rotational transition, whereas in the REMPI-PES spec-
trum for NЈϭ5, only transitions up to ⌬NϭϮ2 are ob-
served. Secondly, the asymmetry between ⌬Nϭϩ2 and ⌬N
ϭϪ2 transitions, obvious in both spectra, is slightly more
J. Chem. Phys., Vol. 104, No. 2, 8 January 1996
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526
Milan, Buma, and de Lange: Spectroscopy of SHϩ
ionization continua under the influence of the pulsed field. A
recent study on NO2 ͑Ref. 7͒ has shown that these rotational
interactions can give rise to sharp substructure on ZEKE-PFI
peaks. The width of such structures is no longer determined
by the magnitude of the electric field pulse, but by the prin-
cipal quantum numbers of the autoionizing low-n Rydberg
states. The narrowness of some ‘‘forbidden’’ ⌬NϭϪ3 and
Ϫ4 rotational transitions seen in our experiment, whose
widths are considerably smaller than the widths of the other
lines in the spectra, could be explained with a similar
mechanism.7,8
We thus see that rotational interaction, either of discrete-
continuum or discrete–discrete nature, is not of dominant
importance in our experiments, since the intensity perturba-
tions in our ZEKE-PFI spectra do not prevail to such an
extent that the major characteristics of the ionization process
observed in the REMPI-PES spectra are eliminated. An ex-
ample of a situation where the latter does occur is found in
the ZEKE-PFI spectra of HCl and HBr.5,9 In these molecules
rotational interactions dominate the rotational branching ra-
tios and, as a result, REMPI-PES and ZEKE-PFI spectra
hardly show any resemblance.
occurs preferentially by absorption of a third photon after the
two-photon excitation step, rather than through electrostatic
autoionization.25
The role of high-n Rydberg depleting processes has been
investigated as well in another type of experiment. In a non-
ZEKE experiment we have compared the contribution of the
various autoionization processes to that of direct ionization.
To this purpose, the intermediate rotational level NЈϭ7 was
selected via the S1(11/2) rotational transition. While the
probe laser scanned the various ionization thresholds, the
photoelectrons deriving from competing ionization processes
were selectively monitored. Below and above the first rota-
1
tional level of the excited a ⌬ ionic state, the initially popu-
lated Rydberg states may couple to the ionization continua of
3
the lower-lying X ⌺Ϫ ground state of the ion leading to an
3
ion in its X ⌺Ϫ ground state and a free electron with a
kinetic energy of ϳ1.2 eV. The other possibility of generat-
ing photoelectrons with a kinetic energy of ϳ1.2 eV, viz.,
1
direct ionization via the [a ⌬]3d␲ 2⌽ ( Јϭ0) Rydberg
v
state into the continua of the X ⌺Ϫ ground ionic state, is not
3
considered here since our previous REMPI-PES study
showed no indication of such a photoionization process.27
The lower panel in Fig. 3 displays the spectrum obtained
by monitoring these photoelectrons deriving from electro-
static autoionization. There is ample evidence for a high den-
sity of discrete resonances, which are known to cause
anomalies in ZEKE-PFI spectroscopy,7 even though these
resonances do not seem to converge upon specific ionization
thresholds. In these experiments our initial state is the
Another kind of core-to-electron energy transfer process
that needs to be considered concerns electrostatic autoioniz-
ation. The ionic core associated with the accessible ZEKE-
1
PFI states is the excited a ⌬ ( ϩϭ0) state of the SHϩ
v
͑SDϩ͒ ion. Hence, accessible discrete states could be subject
to autoionization into the continua of the lower-lying X ⌺Ϫ
3
ground state of the ion. The photoelectrons with a kinetic
energy of ϳ1.2 eV derived from such an electrostatic auto-
ionization process are, however, quite distinct from the near-
zero-kinetic-energy ZEKE-PFI electrons. Electrostatic auto-
ionization may therefore result in an overall depletion of the
ZEKE-Rydberg states, but it cannot influence the rotational
branching ratios in our ZEKE-PFI experiments. It is clear
that the electrostatic coupling between the ZEKE-Rydberg
1
[a ⌬]3d␲ 2⌽ state. It seems highly improbable that one-
photon excitation from this state would be able to access
1
Rydberg states with a b ⌺ϩ excited ionic core, since this
would in general involve a two-electron transition. More-
over, in such a picture the density of resonances observed is
too high when we bear in mind that we start from a single
2
rotational level in the ⌽ state. We therefore conclude that
3
states and the continua of the X ⌺Ϫ ionic ground state is
the resonances should be assigned to Rydberg states converg-
1
small: the lifetime of these high-n Rydberg states is large
enough to survive the delay of ϳ60 ns before the pulsed
electric field is applied. On the other hand, the delay times
possible in the present experiments are significantly shorter
than those used in similar ZEKE-PFI experiments on HCl
and HBr in the ‘‘magnetic bottle’’ spectrometer.5,9 This indi-
cates that electronic autoionization and/or predissociation
processes ͑vide infra͒ do influence the overall lifetime of the
high-n Rydberg states employed for ZEKE-PFI.
ing upon the a ⌬ excited ionic core.
If the photon energy of the probe laser exceeds the low-
1
est ionization threshold of the a ⌬ excited ionic state, rota-
tional autoionization becomes possible, because the discrete
states can now couple to the various ionization continua of
1
the lower-lying rotational levels associated with the a ⌬
excited ionic state. This process is clearly demonstrated in
the top spectrum of Fig. 3, where electrons are monitored
with near-zero kinetic energy. It is observed that rotational
autoionization starts as soon as the lowest rotational level of
The minor role of electrostatic autoionization for SH
1
1
͑SD͒ Rydberg states with an a ⌬ core has also been con-
the a ⌬ state, Nϩϭ2, is reached. The prevailing photoion-
cluded from other experiments in which we could observe
rotationally well-resolved resonance-enhanced excitation
spectra of SH ͑SD͒ Rydberg states with excitation energies
above the lowest ionization threshold.25,27 First, if electro-
static autoionization would proceed on a short timescale,
these excitation spectra would have exhibited an extensive
lifetime broadening. Secondly, ͑2ϩ1͒ REMPI-PES spectra
ization selection rule ͉⌬N͉р2 would imply that the Rydberg
states observed above the Nϩϭ2 rotational threshold con-
verge upon rotational thresholds with Nϩу5. Indeed, the
resonances observed between the Nϩϭ2 and Nϩϭ5 thresh-
olds seem to converge by and large to the Nϩϭ5 threshold.
The same selection rule makes direct ionization into the con-
1
tinua of rotational levels associated with the a ⌬ excited
1
obtained for ionization via the [a ⌬]5p␲ 2⌽ Rydberg state
ionic state only allowed when the rotational threshold of
Nϩϭ5 is reached. Accordingly, it is observed that at the
Nϩϭ5 threshold, the production of slow electrons is in
of the SH ͑SD͒ radical, which has an excitation energy ex-
ceeding the lowest ionization threshold, show that ionization
J. Chem. Phys., Vol. 104, No. 2, 8 January 1996
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Milan, Buma, and de Lange: Spectroscopy of SHϩ
527
ACKNOWLEDGMENTS
creased dramatically. This sharp rise in ionization yield
might be attributed to two different scenarios. In the first one
rotational autoionization is very inefficient below the Nϩϭ5
threshold, making the direct ionization channel dominate the
ionization characteristics above Nϩϭ5. In that case we
would expect the electronic autoionization channel to show
an equivalent drop. What we observe in this channel, how-
ever, is that the intensity of the resonances decreases slightly
between the Nϩϭ2 and Nϩϭ5 thresholds, and gradually
dies away above the Nϩϭ5 threshold without the expected
sharp drop. In the second scenario, Rydberg states in the
present energy region may be subject to predissociation, a
decay channel which will become unimportant for energies
where direct ionization is possible. This would similarly lead
to a sharp rise in the ionization yield at Nϩϭ5, but without
an accompanying sharp drop in the electronic autoionization
channel.
The authors thank K. Wang and V. McKoy ͑California
Institute of Technology͒ for helpful discussions, and the ref-
eree for constructive comments. The financial support from
the Netherlands Organization of Scientific Research ͑NWO͒
is gratefully acknowledged.
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J. Chem. Phys., Vol. 104, No. 2, 8 January 1996
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