Laser excitation and photoionization spectroscopy of the AO+ and B1 states of indium monoiodide: Ground state dissociation energy and photodissociation yield of In(6s2S1/2)

Article abstract of DOI:10.1063/1.479377

The dissociation energy of the InI ground state (X¹Σ⁺(O⁺)) and the relative yield of In(6s ²S_1/2) from the photodissociation of InI by sequential two-photon absorption have been measured by laser-indu

Full text of DOI:10.1063/1.479377

Laser excitation and photoionization spectroscopy of the A O + and B1 states of indium
monoiodide: Ground state dissociation energy and photodissociation yield of In (6s 2 S
1/2 )
K. K. King, C. M. Herring, and J. G. Eden
Citation: The Journal of Chemical Physics 111, 931 (1999); doi: 10.1063/1.479377
View online: http://dx.doi.org/10.1063/1.479377
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 111, NUMBER 3
15 JULY 1999
؉
Laser excitation and photoionization spectroscopy of the AO and B1
states of indium monoiodide: Ground state dissociation energy
2
and photodissociation yield of In„6s S …
1/2
a)
b)
c)
K. K. King, C. M. Herring, and J. G. Eden
Everitt Laboratory, Department of Electrical and Computer Engineering, University of Illinois, Urbana,
Illinois 61801
͑
Received 31 July 1998; accepted 23 April 1999͒
1
ϩ
ϩ
2
The dissociation energy of the InI ground state (X ⌺ (O )) and the relative yield of In(6s S1/2)
from the photodissociation of InI by sequential two-photon absorption have been measured by
laser-induced fluorescence and excitation spectroscopy in the 390–431 nm region. Laser excitation
ϩ
spectroscopy and photoionization experiments demonstrate that the XO dissociation energy is
Ϫ1
2
5 340Ϯ90 cm , which agrees with the value suggested by Vempati and Jones ͓J. Mol. Spectrosc.
127, 232 ͑1988͔͒ if the InI ground state is covalent. Seven previously unreported vibrational bands
of the InI(B←X) transition have been observed and the molecular emission spectra indicate that the
3
B ⌸ (1) state is predissociated above vϭ6. Splitting between the fine structure levels of the In
1
2
np P
(15рnр17) states has also been measured by fluorescence suppression spectroscopy,
1
/2,3/2
following the three photon excitation of InI in the 450–470 nm interval. © 1999 American
Institute of Physics. ͓S0021-9606͑99͒01827-9͔
2
2
I. INTRODUCTION
In(5p P1 )ϩI(5p P ). Presuming, however, that the
/2
3/2
11
ground state is ionic, Vempati and Jones concluded that the
AO and B1 states are also derived from In(5p P1/2)
ϩI( P3 ) and the ground-state dissociation energy rises to
ϳ27 500 cm . The latter scenario was favored in Ref. 11 on
the basis of molecular orbital correlation arguments.
Spectroscopic constants for electronic states of InI and
other Group IIIA metal–halide diatomics are of interest for
several reasons, one of which is the role of these species in
gas discharge lamps. Furthermore, measurements of rela-
tive photodissociation yields of metal atoms in specific ex-
In 1934, Wehrli and Miescher1,2 reported the observation
of three excited states of InI. The two lowest lying of these
are bound and were labeled A ⌸ and B ⌸ , while transi-
ϩ
2
2
/2
3
3
Ϫ1
0
1
1
tions from ground to the third state, denoted C ⌸ , give rise
1
to a broad continuum in the ultraviolet (␭ϳ318 nm). From
microwave absorption experiments, Barrett and Mandel3
subsequently determined the rotational constants for the
ground states of the monohalides of aluminum, gallium, and
indium. Schenk et al. extended the work of Ref. 3 by in-
cluding hyperfine structure in the analysis of the InI rota-
tional spectrum. With the Kitt Peak Fourier-transform spec-
12
4
13
cited states or metal–halogen atomic ion pairs are vital to
14
determining the practicality of thin film deposition or
laser15 schemes. In this paper, the results of laser spectro-
scopic experimental studies of the A and B states of InI are
reported. Laser-induced fluorescence and excitation spectros-
copy in the 390–431 nm region reveal seven previously un-
reported B←X(vЈ–vЉ) bands, and the spectra suggest that
the B1 state is predissociated above vЈϭ6. Comparison of
B,A←X absorption spectra with the wavelength-dependent
5
trometer, Davis and Pecyner measured the rotational line
positions for 234 vibrational bands of the A,B←X transi-
Ϫ1
tions in absorption in the 22 285–25 322 cm region and
determined vibrational and rotational spectroscopic constants
for the X, A, and B states.
Despite the availability of high-resolution absorption
measurements, a significant issue yet to be resolved for InI is
the dissociation energy of the ground state (X ⌺ (O )).
Primarily on the basis of thermochemical data, Barrow sug-
1
ϩ
ϩ
6
2
7
relative yield of In(6s S ) shows that the upper limit on
1/2
Ϫ1
Ϫ1
De for the InI(X) state is 25 429 cm . Photoionization spec-
tra acquired in the vicinity of the I photodetachment thresh-
old reveal that a conservative lower limit on D (X) is 25 143
cm . These values support the proposal by Barrow that the
InI ground state is covalent and derived from In( P1/2)
ϩI( P3/2).
The photodissociation spectra also exhibit a strong de-
pendence of In(6s S1/2) yield on the rotational quantum
gested that D (X)ϳ27 600 cm , which is 7%–15% larger
0
Ϫ
8
than the values estimated by Bulewicz et al. and Wehrli and
1
e
Miescher from flame photometry and spectroscopic experi-
Ϫ1
7
ments, respectively. Vempati and Jones9
–11
determined the
2
rotational constants of the A and B states from an analysis of
microwave discharge emission spectra and concluded that
the InI(X) state dissociation energy is 25 296 cm if XO is
covalent and correlates, in the separated atom limit, with
2
Ϫ1
ϩ
2
number of the intermediate ͓B1(vЈ,JЈ)͔ state, a characteris-
tic that is not present in the photoionization spectra. Finally,
a͒Present address: Cutting Edge Optronics, Inc., 20 Point West Blvd., St.
Charles, MO 63301.
Present address: Caviton, Inc., 702 Scottswood Dr., Urbana, IL 61802.
splitting between the fine structure components of the
2
b͒
c͒
np P (nр17) states of In and the dipole-forbidden
J
2
2
2
Electronic mail: jgeden@uiuc.edu
ns S1 ,md D
←6s S
3/2,5/2
(15рnр25, 13рmр25͒
/2
1/2
0021-9606/99/111(3)/931/7/$15.00
931
© 1999 American Institute of Physics
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1

932
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
King, Herring, and Eden
transitions have been observed by fluorescence suppression
2
spectroscopy of the In(6s S1/2) species, following three-
photon excitation of InI in the 450–470 nm spectral region.
II. EXPERIMENTAL APPARATUS AND TECHNIQUES
All experiments were conducted under static fill condi-
tions with InI-containing quartz optical cells. After the cells
were evacuated to below 10Ϫ6 Torr and outgassed, natural
abundance InI was introduced and the cell sealed. For most
of the experiments described here, the cells were operated at
a temperature of 650Ϯ5 K, which corresponds to an InI
16
Ϫ3
number density of 10 cm . Radiation from an Nd:YAG
Ϫ1
laser-pumped dye laser having a linewidth of ϳ0.1 cm was
directed along the axis of the cell and fluorescence observed
at right angles by a 0.25 m monochromator and a photomul-
tiplier. Experiments were conducted in which either atomic
In or laser-induced InI(B,A→X) emission was detected. The
dye laser wavelength was calibrated by Ne optogalvanic
FIG. 1. Absorption spectrum of InI in the 300–425 nm region. The C1
ϩ
←XO continuum is saturated between ϳ324 and 326 nm and the highly
ϩ
structured bands are associated with AO , B1←X transitions of the mol-
Ϫ1
ecule. For these data, the cell temperature was 650 K, which corresponds to
spectroscopy to an estimated uncertainty of Ϯ0.1 cm . The
16
Ϫ3
͓
InI͔Ӎ10 cm .
nominal dye laser pulsewidth is 8 ns and the pulse energy
was, unless noted otherwise, maintained at ϳ1 mJ. A boxcar
integrator sampled the photomultiplier output and the entire
experiment was under the control of a computer.
Survey absorption spectra of InI were obtained at a reso-
lution of 0.2 nm with a Xe arc lamp providing the source
continuum and a 1 m spectrograph ͑having a reciprocal dis-
persion of 0.82 nm/mm in first order͒ equipped with a diode
array. The spectral dependence of the lamp output was ac-
counted for by sampling the beam incident upon and trans-
mitted by the optical cell with fiber bundles and an optical
multichannel analyzer.
Photoionization experiments were also conducted in
which the optical cell was replaced by a proportional counter
which is simply a Pyrex tube, 32 cm in length, having a
cylindrical diode. The anode is a 150 ␮m diameter nickel
wire suspended along the axis of a cylindrical stainless-steel
cathode by Ni–W feedthroughs. The cell windows are
Suprasil-grade quartz and, after introducing InI, the counter
was backfilled with 150 mTorr of He. A buffer amplifier was
inserted into the signal path between the ionization cell and
boxcar amplifier. Bias voltages between ϳ200 and ϳ350 V
were required to operate the diode in the linear portion of its
response ͑signal vs voltage͒ characteristic.
A series of experiments was conducted in which molecu-
lar and atomic excitation spectra were obtained. Molecular
spectra were acquired predominantly for the B1 state which
involved monitoring the relative intensity of a particular B
→
X(vЈ,vЉ) emission line as the dye laser wavelength was
scanned. This experimental approach is essentially a sensi-
tive means for acquiring B←X absorption spectra. Similarly,
2
atomic excitation spectra for the In 6 S state were ob-
1/2
2
tained by recording the relative intensity of the 6s S
1/2
2
→
5p P3 atomic line ͑451.1 nm͒ as a function of dye laser
wavelength. Results representative of more than 20 spectra
acquired in the 390–410 nm interval are illustrated in
/2
Figs. 2–5.
The In(6s 2S1/2) excitation spectrum in the 390–410 nm
interval is illustrated in Fig. 2. Upon examining the data, it is
ϩ
evident that features attributable to the AO state are con-
III. RESULTS AND DISCUSSION
2
A. Molecular „B—X… and atomic „In„6 S1/2……
excitation spectra
Figure 1 shows the absorption spectrum for InI at 650 K
in the 300–425 nm region. The highly structured B1
ϩ
ϩ
ϩ
ϩ
←
XO and AO ←XO bands as well as the C1←XO
continuum dominate the spectrum. The latter is clearly satu-
ϩ
rated and the six strongest features in the B1←XO spec-
trum ͑lying between 399 and 403 nm͒ are all members of the
vЈϪvЉϵ⌬vϭ0 progression. Simulations of the experimen-
tal A,B←X absorption spectra, calculated from the spectro-
scopic constants of Refs. 5 and 11, match the data of Fig. 1
and higher-resolution measurements (⌬␭Ͻ0.1 nm, cf. Fig.
FIG. 2. Overview of the laser excitation spectrum of In(6s 2S1/2) in the
violet ͑390–410 nm͒. Note the clear, red-degraded vibrational bands of the
B←X transition lying between ϳ390 and 394 nm. B←X vibrational bands
associated with other prominent features in the spectrum are also indicated.
8͒ quite well.
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J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Spectroscopy indium monoiodide
933
FIG. 5. Comparison of: ͑a͒ the molecular excitation spectrum recorded by
monitoring B(vЈϭ3)→X(vЉϭ5) fluorescence at 406.8 nm, and ͑b͒ the
2
In(6 S ) excitation spectrum in the 395–403 nm region.
1/2
the In(6s 2S1/2) excitation spectrum in the 391–398 nm re-
gion along with the molecular excitation spectrum obtained
by monitoring only the B→X(vЈ–vЉϭ1–1) transition at
399.9 nm. The strongest feature in the molecular spectrum,
the ͑1–0͒ band, has a profile consistent with previous absorp-
FIG. 3. ͑a͒ InI molecular fluorescence spectrum obtained over the 391–398
nm interval by monitoring the B←X(vЈ–vЉ)ϭ(1–1) transition at 399.9
2
nm, and ͑b͒ the atomic In(6s S ) excitation spectrum over the same spec-
1
/2
tral region. Several of the most prominent B←X bands are labeled.
1,10
tion and emission experiments.
Because the rotational
ϩ
constants (B ) for the XO and B1 states differ by
e
g
Ϫ3
Ϫ1
Ͻ10 cm ͑Refs. 10, 11͒, additional heads often appear in
the P, Q, or R branches of vibrational bands at large J. The
extra head evident here ͓Fig. 3͑a͔͒ is also present in the
spicuous by their absence. Only intermediate resonances as-
sociated with the B1 level are observed, and several are la-
beled in Fig. 2. Notice, however, that well-developed
vibrational bands are present only in the short wavelength
portion of the spectrum. For ␭տ395 nm, the features are, in
general, considerably sharper, and beyond ϳ400 nm the
spectrum rapidly diminishes in intensity.
͑
1–1͒ band. Of particular interest, however, is the structure
2
in the photodissociation (In 6 S yield͒ spectrum and its
1/2
correlation with the ͑1–0͒ absorption ͑excitation͒ profile.
2
Note that maximum In 6 S yield is sharply peaked and
1/2
2
does not coincide with maximum absorption. Rather, In
excited-state production is clearly enhanced by high J inter-
mediate states of the R ͑and possibly Q͒ branch. Similarly,
when the B(vЈϭ0) excitation spectrum ͑not shown͒ was
obtained by monitoring the B→X(0–2) transition at 405.0
nm, peak ͑0–0͒ P-branch absorption was observed to coin-
A detailed comparison of the In(6 S1/2) excitation spec-
trum with corresponding portions of the B1(vЈ) excitation
spectra reveal several trends that are illustrated in Figs. 3–5.
Parts ͑b͒ and ͑a͒ of Fig. 3, for example, show ͑respectively͒
2
cide with a small suppression in the In(6 S) yield and, con-
versely, maximum yield is associated with high J values of
the R branch. Almost identical results were obtained for the
͑1–1͒ band which was observed by monitoring the ͑1–0͒
transition at 397.0 nm. Similar behavior is observed else-
where as evidenced at longer wavelengths ͑401–411 nm͒ by
the spectra of Fig. 4. In absorption, the B←X(1–2) and
͑
1–3͒ bands exhibit the well-known violet-degraded profiles,
2
but additional peaks appear in the In 6 S yield spectra, dem-
onstrating that photodissociation yield is dependent upon J.
Higher vЈ states were also examined and illustrate these ef-
fects most clearly. Figure 5, for example, shows the atomic
and molecular excitation spectra for several (3–vЉ) band
heads. As discussed by Vempati and Jones,10 shading of the
B←X bands reverses for 2рvЈр3 owing to the rapid varia-
tion of B with vЈ. Consequently, the (3–vЉ) bands are red
vЈ
degraded but it is again the influence of the rotational quan-
tum number on the atomic excitation spectra that is the most
intriguing aspect of these spectra. Consider the lower ͑atomic
FIG. 4. Spectra similar to those of Fig. 3 over the 401–411 nm region. The
molecular laser-induced fluorescence spectrum was acquired by monitoring
the B(vЈϭ1)→X(vЉϭ1) transition.
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934
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
King, Herring, and Eden
2
FIG. 6. In(6 S1/2) excitation spectrum recorded at a dye laser resolution of
Ϫ1
ϳ0.1 cm in the 402–411 nm region. All of the labeled vibrational band
features are associated with B←X transitions.
excitation͒ spectrum of Fig. 5, which shows the presence of
three distinct features associated with the (3–vЉ) bands
shown. The middle feature of the three for the ͑3–1͒ and
FIG. 7. Partial energy level diagram for InI illustrating the sequential ab-
͑
3–3͒ transitions of Fig. 5͑b͒, for example, peaks at ϳ395.91
ϩ
sorption of two pump photons when AO is the intermediate state. The
Ϫ1
2
ϩ
Ϫ
and ϳ401.48 nm, respectively, or several cm to the red of
the corresponding Q branch maxima in the molecular spectra
limits for production of In(6s S1 ) and ion pairs (In –I ) by photodisso-
/2
ciation of InI are also shown. For convenience, the X, A, and B states are
represented by Morse potentials and the spectroscopic constants of Refs. 5
and 11. The lowest horizontal dashed line represents the separated atom
͓Fig. 5͑a͔͒. Lying still further ͑ϳ2 Å͒ to the red is a third
feature, apparently associated with high J transitions in the P
branch. One concludes, therefore, that each of the features in
the In* yield spectrum corresponds to P or Q ͑and, presum-
ϩ
limit for the XO state.
1
6
ϩ
ably, R as well͒ branch transitions of high J. Notice also
that, as one would expect, the widths of the three peaks as-
sociated with the (3–vЉ) bands increase according to the
R→Q→P sequence. Although the features are weaker, the
same behavior is observed for the ͑1–0͒, ͑2–1͒, and ͑2–2͒
bands of Fig. 5, as well as the ͑1–4͒ band of Fig. 4.
An upper limit for the ground (XO ) dissociation en-
ergy can be estimated by noting that the B←X(0–2) tran-
2
sition is clearly present in the In(6 S ) yield spectrum, but
1/2
the ͑1–4͒ band is weak and features attributable to ͑0–3͒
transitions are absent, despite the fact that the ͑1–3͒, ͑2–4͒,
and ͑2–5͒ bands are strong. Consider first the partial energy
level diagram for InI illustrated in Fig. 7. The X, A, and B
states are represented by Morse potentials for which the
spectroscopic constants are taken to be those of Refs. 5 and
11. Although Rydberg–Klein–Rees ͑RKR͒ potentials are
also available ͑Ref. 11͒, Morse potentials are more than ad-
equate for illustrative purposes. The C1 state is assumed to
be a Born potential and the limits associated with the pro-
The data of Figs. 2–5 provide a ruler with which the
ϩ
dissociation energy of the XO state may be measured. The
2
strong dependence of the In(6s S1/2) yield on J reflects the
fact that, as the pump wavelength is decreased, higher rota-
tional energies are required to provide the energy necessary
2
to access the In(6 S1/2)ϩI exit channel. It is no surprise,
therefore, that the features in the atomic excitation spectrum
become increasingly narrow as the pump wavelength moves
2
ϩ
Ϫ
duction of In(6s S1 )ϩI and In –I ion pairs ͑Ref. 13͒ are
/2
2
to the red. Therefore, an examination of the In(6 S1/2) exci-
also shown. Since the energy available to access the neutral
2
tation spectra at higher resolution allows one the opportunity
exit channel In(6 S)ϩI is twice the photon energy plus
ϩ
to determine the XOg dissociation energy to within the
G(X,vЉ), then estimating D (X) is straightforward. For the
e
Ϫ1 10,11
ground-state vibrational energy (␻eXХ177 cm ).
Fig-
key (B←X) vibrational transitions of Fig. 6, the energies
available for photodissociation when exciting that particular
transition are ordered as (3–5)Ͼ(2–4)Ͼ(1–3)Ͼ(2–5)
Ͼ(0–2)Ͼ(1–4)Ͼ(0–3). In other words, the sequential
absorption of two photons via the B←X(1–3) transition ter-
minates at a slightly higher energy than does pumping the
B←X(0–2) band. Therefore, the strength of the features in
Fig. 6 attributed to the B←X(1–3) and ͑2–5͒ transitions
and the weakness of the ͑0–2͒ and ͑1–4͒ features suggest
that the total energy available when driving the ͑0–2͒ band
is immediately above the energy threshold for accessing
2
ure 6 shows the atomic 6 S1 excitation spectrum recorded
/2
in the 402–411 nm region with a dye laser resolution of 0.1
Ϫ1
cm and a peak intensity more than an order of magnitude
lower than that for the data of Figs. 2–5. Both A–X and
B–X transitions lie in this spectral interval, but all of the
peaks in Fig. 6 are attributable to B←X bands. Several A
5
,9
2
←
X bands also appear in the In(6 S ) yield spectrum,
1/2
but only as suppressions. A few of these are indicated by
asterisks in Fig. 6. Several of the B←X bands identifiable in
Fig. 6 are also labeled.
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J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Spectroscopy indium monoiodide
935
FIG. 8. Absorption ͑a͒ and photoionization spectra for InI in the 395–402
nm region. The photoionization spectrum has not been corrected for the
variation of dye laser pulse energy with wavelength.
the In(6 2S1/2)ϩI photodissociation limit. Consequently,
ϩ
the dissociation energy D (XO )Ͻ2
˜
␯
(0–2)ϩG(vЉϭ2)
e
FIG. 9. Comparison of the relative photoionization and In(6 2S1/2) yields
between 404.0 and 405.5 nm showing the enhancement in photoion produc-
ϭ25 429 cmϪ1, where
˜␯
) (0–2)
1/2
Ϫ
˜␯
2
is taken to be the
In(6
S
2
peak of the P branch and the In(6 S1/2) state lies at 24 373
tion yield associated with A–X(2,0) transitions. The location of the thresh-
Ϫ
Ϫ1
old for photodetachment of I is also indicated. It should also be noted that
cm . It should be mentioned that, although this simple ex-
pression ignores the potential barrier associated with the dis-
sociative In(6 S1 )ϩI( P3/2) potential ͓i.e., V(ReX
the ionization spectrum is saturated; the resonances associated with
A–X(2,0) transitions are, in reality, more than an order of magnitude stron-
ger than any other feature in the photoionization spectrum.
2
2
)
/2
–
V(ϱ)], the upper limit is, nevertheless, valid. Also, this
value for D (X) is remarkably close to the lower of the two
e
11
cal to the absorption band profiles in this spectral region and,
in particular, the sharply peaked structures corresponding to
large J that were observed in the In(6 S1/2) yield spectra are
absent in the photoionization spectrum. At longer wave-
lengths ͑403–411 nm͒, A–X bands are now present and the
values proposed by Vempati and Jones and will be dis-
cussed in more detail in the next section. Similarly, in view
of the fact that features associated with the B(0–3) transi-
2
tion are either extremely weak or not present in Figs. 2 and
Ϫ1
6
, the lower limit for D is calculated to be 25 253 cm . On
e
⌬
vϭϩ1,ϩ2 sequences are dominant.
the basis of these considerations, we conclude that D (X)
e
Ϫ1
The most interesting features in the photoionization
ϭ25 340Ϯ90 cm
.
spectra are structures corresponding to the P branch of the
A–X(2–0) transition that lie at 404.5 and 404.8 nm. The
ionization current produced at these wavelengths exceeded
that produced elsewhere by more than an order of magni-
Before leaving the excitation spectra, it should be men-
tioned that seven previously unreported B←X bands ͓͑3,0͒,
͑
4,0͒, ͑4,1͒, ͑5,1͒, ͑5,2͒, ͑6,2͒ and ͑6,3͔͒ have been observed
in these experiments, and several are identified in Fig. 3. The
position of each bandhead is consistent with the X and B
molecular constants of Refs. 10 and 11. No bands for which
vЈϾ6 were observed in the present work or Ref. 5, which
suggests the possibility that higher B(1) vibrational states
are predissociated.
2
tude. Figure 9 presents a comparison of the In(6 S1/2) yield
and photoionization spectra in the 404–405.5 nm interval. It
should first be noted that, because the photoionization spec-
trum has intentionally been saturated, the features lying at
Ϫ
wavelengths below the I photodetachment threshold at ␭
ϳ405.2 nm ͑Ref. 18͒ are considerably stronger than is im-
plied by Fig. 9. Because of the dependence of the cylindrical
diode current on particle mobility, the diode response to
B. Photoionization spectra
Although H a¨ lg17 reported the photoionization of InI in
Ϫ
electrons is greater than that for anions such as I and, there-
13
1
943 and Geohegan et al. studied by microwave tech-
fore, the experiment is considerably more sensitive to the
A–X(2,0) resonances than the A–X(3–1) feature, despite
the fact that the absorption cross section is approximately the
same for both vibrational transitions.
ϩ
Ϫ
niques the In –I ion pair yield resulting from the absorp-
tion by InI of a single 6.4 eV (␭ϭ193 nm) photon, the
photoionization spectra of the molecule have, to our knowl-
edge, not been explored previously. In these experiments,
measurements of the relative negative charge collected by
the proportional counter ͑cylindrical diode͒ were made as the
laser wavelength was tuned between 395 and 431 nm. As
illustrated in Fig. 8, photoionization spectra recorded in the
Of particular interest, however, is the lack of correspon-
dence between features in both spectra of Fig. 9. All of the
prominent features in the photoionization spectrum are asso-
ciated with A–X(2–0) and ͑3–1͒ transitions, and the nota-
tion for labeling the bandheads is adopted from Wehrli and
Miescher.1 Several of the weaker peaks in the photoioniza-
tion spectrum near 405 nm may arise from the B
,19
395–402 nm region generally replicate the InI absorption
spectrum. Band shading and structure are essentially identi-
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936
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
King, Herring, and Eden
ϩ
Ϫ
→
X(0–2) band but, in general, the In –I pair yield shows
little influence from B→X transitions. In the same vein, the
2
In(6 S ) yield spectrum exhibits virtually no structure as-
1
/2
signable to A–X transitions. One concludes, therefore, that,
in the spectral region studied here, A–X transitions offer
insufficient energy to photodissociate InI ͑yielding the
2
In(6s S1/2) species͒ but metal–halide ion pairs are pro-
duced, whereas the opposite is true if one drives B←X tran-
sitions. When exciting B←X(vЈ–vЉ) bands in the 395–410
2
nm region, production of In(6s S ) predominates over the
1/2
formation of ion pairs. Since the excitation of the A–X(2,0)
ϩ
Ϫ
transition produces In –I ion pairs but not the neutral spe-
2
cies In(6 S), one readily finds that the lower limit for the
ϩ
Ϫ1
XO dissociation energy is ϳ25 143 cm . This estimate is
Ϫ1
FIG.
In(ns S1 np P
10.
Fluorescence
suppression
spectrum
of
ϳ100 cm smaller than, but consistent with, the lower limit
2
2
2
2
,nd D
←6s S , 25рnр15) transitions ob-
/2,
1/2,3/2
3/2,5/2
1/2
suggested in the last section.
2
2
tained by monitoring In(6 S1 →5 P ) spontaneous emission at 410.2
/2
1/2
The value for De proposed here, 25 340Ϯ90 cmϪ1,
agrees well with the estimate by Vempati and Jones of
2
nm. In this wavelength region, production of the 6 S state requires the
1/2
11
absorption of three photons.
ϩ
Ϫ1
D (XO )ϭ25 296 cm , assuming the InI ground state to be
0
ϩ
covalently bonded and the AO , B1, and C1 states are de-
rived from In( P )ϩI( P3/2). As discussed in Ref. 11, this
value for the XO dissociation energy is consistent with
measurements from flame photometry ͑ϳ25 880 cm , Ref.
2
2
multiphoton ionization spectrum of the Column III B alkyls
3
/2
ϩ
Al2͑CH3͒6, Ga͑CH3͒3, and In͑CH3͒3 that were reported by
Hackett, John, and Mitchell,2
1–23
who demonstrated that the
Ϫ1
Ϫ1
absorption of 4–6 photons in the visible completely removes
the methyl ligands and yields a metal ion. Thus, the mul-
tiphonon ionization ͑MPI͒ spectrum of the alkyl in the vis-
ible has the signature of solely the metal atom. In the case of
8
͒, the upper limit of 27 630 cm determined from photo-
ionization experiments ͑Ref. 7͒, and the range in values pro-
Ϫ1
posed in Ref. 1 ͑21 700–27 430 cm ͒. It should also be
11
noted that the estimates of D (X) by Vempati and Jones
0
2
In͑CH ͒ , two-photon ionization of the In 6 S state, reso-
3
3
1/2
were drawn from Morse potential extrapolations and, as the
authors mention, ‘‘...should be considered as upper limits.’’
2
nantly enhanced by the np P states (13рnр30) was
J
observed2 but, to our knowledge, the single-photon forbid-
2
2
2
den, md D3
←6s S
transitions have not been re-
/2,5/2
1/2
ported previously.
C. Fluorescence suppression spectra:
2
2
2
2
S1/2 , P
, D
—6s S transitions of In
1/2,3/2
3/2,5/2
1/2
IV. CONCLUSIONS
For excitation wavelengths beyond ϳ410 nm, populat-
ing the 6 S state of In from InI requires the absorption of
2
2
The relative, wavelength-dependent yield of In(6s S1/2)
from InI, and the InI ground state (XO ) dissociation energy
1
/2
ϩ
three photons. Since the threshold wavelength for photoion-
2
ization of the 6s S1 level by two photons of equal energy
have been measured by excitation spectroscopy and laser-
induced fluorescence in the spectral region that overlaps the
/2
2
is 448 nm, excitation spectroscopy of In(6s S1/2) in the ␭
ϩ
ϩ
Ͼ448 nm region is a convenient tool for examining high
AO , B1←XO transitions of the molecule ͑390–431 nm͒.
Seven previously unreported B←X bands have been ob-
served and the absence of vibrational bands for which vЈ
Ͼ6 suggests that the B1 state is predissociated. The pres-
ence or absence of specific A←X or B←X bands in the
Rydberg states of atomic In while simultaneously probing
2
In(6s S1/2) production. Figure 10 shows the fluorescence
suppression spectrum for indium in the 450–470 nm region
2
2
that was recorded by monitoring 6s S1 →5p P emis-
/2
1/2
2
sion at 410.2 nm. The strongest features correspond to the
In(6 S ) excitation spectrum or photoionization spectra
1/2
2
2
ϩ
dipole-allowed np P1
←6s S transitions, and princi-
permit the XO ground-state dissociation energy to be deter-
/2,3/2
1/2
Ϫ1
pal quantum numbers nу25 are readily observable. Also, the
mined to be D ϭ25 340Ϯ90 cm , which agrees with the
e
2
fine structure splitting in the P states has not been reported
lower value suggested in Ref. 11—the value of D to be
e
ϩ
ϩ
previously for the nϭ15–17 terms of the series, but the
values measured in these experiments ͑shown in Fig. 10͒ are
in agreement with the expression given by Neijzen and
expected if: ͑1͒ XO is covalent, and ͑2͒ the AO , B1, and
C1 states are all correlated, in the separated atom limit, with
2
2
In(5p P3 )ϩI( P3/2). Deducing De from direct spectro-
/2
2
0
Ϫ3
Ϫ1
D o¨ nszelmann: ⌬T ϭ5880n*
͑in cm ͒, where ⌬T is
scopic evidence, rather than from the extrapolation of a
Morse potential vibrational series, yields a value more accu-
rate than those available previously, due in part to the fact
that the result proposed here is not dependent upon the struc-
n
n
2
the P
splitting and n* is the effective quantum num-
/2–3/2
1
ber.
Also noticeable in Fig. 10 are single-photon transitions
2
2
ϩ
terminating on the ns S1 (15рnр25) and md D
ture of the XO state near the dissociation limit. The
/2
3/2,5/2
2
(
13рmр25) states of the atom. These transitions were
In(6s S1/2) relative yield data, particularly in the 395–407
found to quickly vanish as the laser pulse energy was de-
creased, but were observed for principal quantum numbers as
low as mϭ9. The spectrum of Fig. 10 is reminiscent of the
nm region, exhibit a strong dependence upon the B-state ro-
tational quantum number, which reflects the fact that the
photodissociation process is near threshold in this spectral
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J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Spectroscopy indium monoiodide
937
4
5
6
region. Fluorescence suppression spectroscopy in the 450–
B. Schenk, E. Tiemann, and J. Hoeft, Z. Naturforsch. A 25A, 1827 ͑1970͒.
S. P. Davis and R. Pecyner, J. Opt. Soc. Am. B B5, 1995 ͑1988͒.
In Hund’s case ͑c͒ notation, the A, B, and ground electronic states of the
molecule are denoted AO , B1, and XO , respectively, and will be so
labeled throughout the remainder of this paper.
2
4
70 nm region gives the relative In(6s S1 ) yield and has
/2
2
revealed transitions originating from the 6s S state of In
and terminating on np P1
/2,3/2
ϩ
ϩ
1/2
2
2
2
, ns S , and nd D
1/2 3/2,5/2
7
8
Rydberg states having principal quantum numbers larger
R. F. Barrow, Trans. Faraday Soc. 56, 952 ͑1960͒.
E. M. Bulewicz, L. F. Phillips, and T. M. Sugden, Trans. Faraday Soc. 57,
2
than 25. The nd,ns← S transitions are normally forbid-
1/2
921 ͑1961͒.
den by L–S coupling selection rules and appear only at the
9
10
11
S. N. Vempati and W. E. Jones, J. Mol. Spectrosc. 120, 441 ͑1986͒.
S. N. Vempati and W. E. Jones, J. Mol. Spectrosc. 122, 190 ͑1987͒.
S. N. Vempati and W. E. Jones, J. Mol. Spectrosc. 127, 232 ͑1988͒.
J. F. Waymouth, Electric Discharge Lamps ͑MIT Press, Cambridge, MA,
highest pump laser intensities available. Fine structure split-
2
ting within the np P states of In (nϭ15–17) was also ob-
12
served.
1971͒, pp. 206–234.
The focus of these experiments is the study of metal–
halide diatomic excited-state dynamics and, specifically, the
competition between photodissociation, ionization, and mo-
lecular fluorescence in energy disposal. To our knowledge,
these experiments are the first laser spectroscopic studies of
InI, and further experiments in this and structurally similar
metal–halides will be necessary to optimize the production
13
D. B. Geohegan, A. W. McCown, and J. G. Eden, J. Chem. Phys. 81, 5336
1984͒.
͑
1
1
1
4
5
6
D. B. Geohegan and J. G. Eden, Appl. Phys. Lett. 45, 1146 ͑1984͒.
See, for example, R. Burnham, Appl. Phys. Lett. 30, 132 ͑1977͒.
With the currently available experimental resolution, wavelength shifts
between the peak of the R branch in the molecular excitation spectrum and
the maximum of the shortest wavelength feature in the atomic spectrum
are not reliably measurable.
W. H a¨ lg, Helv. Phys. Acta 16, 371 ͑1943͒.
See, for example, C. R. Webster, I. S. McDermid, and C. T. Rettner, J.
Chem. Phys. 78, 646 ͑1983͒.
As mentioned earlier, the small difference between the rotational constants
for the X, A, and B states results in the formation of secondary bandheads
that are denoted in Ref. 1 by primes. Broad bandheads with sharply de-
fined edges resulting from the heads of two P or Q bands in close prox-
imity are designated by the subscript L. These representations have been
followed in labeling the key features of Fig. 9 and the photoionization
spectrum, in particular.
17
ϩ
2
of a desired species such as In or In(6s S1/2).
18
19
ACKNOWLEDGMENTS
The technical assistance of K. Kuehl and K. Voyles is
gratefully acknowledged. This work was supported by the
U.S. Air Force Office of Scientific Research and the Joint
Services Electronics Program.
20
J. H. M. Neijzen and A. D o¨ nszelmann, Physica 106C, 271 ͑1981͒.
1
21
22
23
M. Wehrli and E. Miescher, Helv. Phys. Acta 7, 298 ͑1934͒.
M. Wehrli, Helv. Phys. Acta 7, 611 ͑1934͒; 7, 676 ͑1934͒.
A. H. Barrett and M. Mandel, Phys. Rev. 109, 1572 ͑1958͒.
P. A. Hackett and P. John, J. Chem. Phys. 79, 3593 ͑1983͒.
S. A. Mitchell and P. A. Hackett, J. Chem. Phys. 79, 4815 ͑1983͒.
S. A. Mitchell and P. A. Hackett, Chem. Phys. Lett. 107, 508 ͑1984͒.
2
3
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1