The relaxation from linear to triangular Ag3 probed by femtosecond resonant two-photon ionization

Article abstract of DOI:10.1063/1.479382

We present extended NeNePo (negative to neutral to positive) measurements on the ultrafast dynamics in the ground state of neutral, mass-selected Ag₃ molecules. A vibrational wave packet in the neutral molecule is created with an ultrashort laser pulse by photodetachment of the excess electron from the corresponding mass-selected anion. The subsequent molecular rearrangement is probed by photoionization after a selected time delay. Complementary to our previous investigations of this process, we now use two-photon photoionization via a resonant state in the probe step. Here, a bound-bound excitation to a well-known state followed by one-photon ionization is used instead of the nonresonant bound/free transition into the ionic continuum. Using radiation with wavelengths near 370 nm for resonant ionization, we observe a fast bending motion of the initially linear Ag₃, followed by an ultrafast intramolecular vibrational energy redistribution, interpretable as an intramolecular collision process. The signal shows an apparent loss of vibrational coherence after the collision, which can be explained by the finite temperature of the anionic clusters in our experiment. Additionally, we describe a previously unknown resonance in the linear neutral molecule around 500 nm.

Full text of DOI:10.1063/1.479382

The relaxation from linear to triangular Ag 3 probed by femtosecond resonant two-
photon ionization
Thomas Leisner, Stefan Vajda, Sebastian Wolf, Ludger Wöste, and R. Stephen Berry
Citation: The Journal of Chemical Physics 111, 1017 (1999); doi: 10.1063/1.479382
View online: http://dx.doi.org/10.1063/1.479382
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/111/3?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Resonant two-photon ionization spectroscopy of jet-cooled UN: Determination of the ground state
J. Chem. Phys. 138, 184303 (2013); 10.1063/1.4803472
Resonant two-photon ionization and ab initio conformational analysis of haloethyl benzenes ( Ph C H 2 C H 2 X ,
X = Cl , F )
J. Chem. Phys. 127, 134307 (2007); 10.1063/1.2772612
Femtosecond dynamics of Cu ( H 2 O ) 2
J. Chem. Phys. 122, 054310 (2005); 10.1063/1.1836759
Femtosecond study of Cu ( H 2 O ) dynamics
J. Chem. Phys. 121, 5676 (2004); 10.1063/1.1782176
Resonant two-photon ionization study of jet-cooled amino acid: L-phenylalanine and its monohydrated complex
J. Chem. Phys. 116, 8251 (2002); 10.1063/1.1477452
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.70.241.163 On: Mon, 22 Dec 2014 08:56:54

JOURNAL OF CHEMICAL PHYSICS
VOLUME 111, NUMBER 3
15 JULY 1999
The relaxation from linear to triangular Ag3 probed by femtosecond
resonant two-photon ionization
¨
Thomas Leisner, Stefan Vajda, Sebastian Wolf, and Ludger Woste
¨
¨
Institut fur Experimentalphysik, Freie Universitat Berlin, Arnimallee 14, 14195 Barlin, Germany
R. Stephen Berry
The University of Chicago, Chicago, Illinois 60637
͑Received 6 February 1998; accepted 22 April 1999͒
We present extended NeNePo ͑negative to neutral to positive͒ measurements on the ultrafast
dynamics in the ground state of neutral, mass-selected Ag3 molecules. A vibrational wave packet in
the neutral molecule is created with an ultrashort laser pulse by photodetachment of the excess
electron from the corresponding mass-selected anion. The subsequent molecular rearrangement is
probed by photoionization after a selected time delay. Complementary to our previous investigations
of this process, we now use two-photon photoionization via a resonant state in the probe step. Here,
a bound–bound excitation to a well-known state followed by one-photon ionization is used instead
of the nonresonant bound/free transition into the ionic continuum. Using radiation with wavelengths
near 370 nm for resonant ionization, we observe a fast bending motion of the initially linear Ag3,
followed by an ultrafast intramolecular vibrational energy redistribution, interpretable as an
intramolecular collision process. The signal shows an apparent loss of vibrational coherence after
the collision, which can be explained by the finite temperature of the anionic clusters in our
experiment. Additionally, we describe a previously unknown resonance in the linear neutral
molecule around 500 nm. © 1999 American Institute of Physics. ͓S0021-9606͑99͒01127-7͔
INTRODUCTION
tection of zero-kinetic-energy electrons in coincidence with
the photoions would remove the ambiguity associated with
the spectrum of final energies of the photoelectrons. The cat-
ion yield in such measurements must reflect faithfully any
change in the vertical ionization potential of the molecule
along its trajectory on the neutral potential energy surface, as
well as the occurrence of internal vibrational relaxation
͑IVR͒ at various time scales. However, ZEKE-NeNePo is
rather difficult to realize experimentally, so we chose instead
to use resonant multiphoton ionization for the probe step.
The cross section for the bound–bound transition involved
here effectively determines the photoion yield and should be
as sensitive to geometry changes in the neutral molecule as
the ZEKE photoionization probability. In this contribution
we demonstrate the use of resonant two-photon ionization to
probe the relaxation of neutral Ag3 from the linear configu-
ration of its precursor anion to a highly excited, triangular
geometry. Following Ref. 7, we discuss our results in terms
of a sequence of coherent molecular configuration changes,
an intramolecular collision process, and internal vibrational
energy redistribution.
Femtosecond NeNePo spectroscopy ͑negative to neutral
to positive͒ is a powerful method to probe the dynamics of
ground-state molecules.1–4 It relies on the preparation of a
vibrational wave packet in the neutral molecule by photode-
tachment of the excess electron from the corresponding an-
ion with an ultrashort laser pulse. The evolution of the wave
packet is probed by photoionizing the neutral molecule with
a second laser pulse after a selected, variable time delay. The
method can yield dynamic information when it is applied to
molecules with significantly different anionic and neutral ge-
ometry. The method allows a straightforward implementa-
tion of mass selection of the ions both prior to and after the
charge reversal process.
The experimental feasibility of the method has been
proven using silver trimers and larger silver clusters as
model systems. Since then it has attracted considerable
theoretical5–10 and experimental4 interest. However, a sub-
stantial limitation of the original experiments is its restriction
to nonresonant two-photon photoionization. The photon en-
ergies should be chosen to lie well above the ionization po-
tential of the neutral molecule in order to avoid the interfer-
ence of molecular Rydberg states or autoionizing resonances.
As a recent theoretical analysis has elucidated,7 under such
conditions the ion yield will only partially mirror the mo-
lecular dynamics in the ground state. Much information is
lost by the integration over the unknown energy of the free
electron.
EXPERIMENT
Our experiments were performed in a guided ion beam
apparatus which consists of a sputtering cluster ion source
and four subsequent radio frequency ͑RF͒ quadrupole assem-
blies ͑Fig. 1͒.
Four beams of Xeϩ ions are created by a modified cold
reflex discharge ion source ͑CORDIS͒. This device employs
a magnetically confined plasma which is heated directly by
tantalum filaments from which the ions are extracted and
The authors of Ref. 7 therefore consider combining this
method with zero-kinetic-energy ͑ZEKE͒ photoionization, in
what might naturally be called ZEKE-NeNePo. Here the de-
0021-9606/99/111(3)/1017/5/$15.00
1017
© 1999 American Institute of Physics
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.70.241.163 On: Mon, 22 Dec 2014 08:56:54

1018
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Leisner et al.
FIG. 1. Schematic diagram of the ex-
perimental apparatus.
accelerated by a three-element electrostatic lens. Details and
specifications of the original CORDIS are given in Ref. 11.
We have modified the extraction electrodes to hold four
separate ion lenses in a square arrangement. Thereby we ex-
tract four independent ion beams, each of which deviates by
an angle of 5° from the axis of the ion source. The highest
ion energies and currents obtained are 20 keV and 10 mA per
beam. After passing through a differential pumping stage,
each of the beams hits a quadratic silver sheet (lϭ25 mm).
The particles sputtered from the surface consist of atoms,
ions, and neutral and ionic clusters.
which is applied collinearly to the trap axis. Any cationic
products are no longer confined inside the trap but are ex-
tracted by the entrance and exit lens. The cluster cations
leaving the ion trap via the exit lens are mass analyzed in a
second quadrupole mass filter (Q3). Their yield is measured
by an off-axis conversion dynode detector as a function of
the wavelength, the duration, and the energy of the laser
pulses and their respective delays.
Neutral clusters produced from the anions by the pump
or the probe pulse but not ionized subsequently, are no
longer trapped and leave the quadrupole due to their velocity
at the time of neutralization. It may require several hundred
microseconds until all neutral molecules have left the ion
trap. This limits the repetition rate at which this experiment
can be performed. We therefore used a Nd:YLF laser-
pumped regenerative titanium:sapphire amplifier running at
1 kHz as a source for the ultrashort laser pulses. It is seeded
with the pulses from a titanium:sapphire oscillator and pro-
duces pulses of about 110 fs duration and typically 400 ␮J
per pulse. Fifteen percent of the power is split off each pulse,
its diameter is reduced with a telescope to 0.5 mm, and its
frequency is doubled before it is sent to the apparatus for
photodetachment. The remaining 85% of the power is used
to pump an optical parametric generator/amplifier, whose
output is tunable between 1570 and 1200 nm. At this point
the light beam is either frequency-doubled two times to pro-
vide a femtosecond ultraviolet ͑UV͒ source tunable between
385 and 300 nm or put into an optical parametric generator
for sum-frequency generation between pump and signal
pulses to reach the wavelength range between 530 and 475
nm. These pulses, with a duration of about 120 fs, are also
collimated to a diameter of 0.5 mm and spatially overlapped
with the photodetachment laser beam at 400 nm which is
passed through a computer-controlled delay line. Both are
introduced collinearly with the quadrupole arrangement
through the detector flange of the quadrupole system.
To monitor the temporal and spatial overlap between
pump and probe pulse in the region of interaction with the
ions, we use a copper electrode which can be positioned in
the ion trap under measurement conditions. The photoelec-
tron current from this electrode, produced by two-photon
photoemission, is measured with a picoammeter as a func-
tion of the delay. It exhibits a pronounced maximum when
both pulses overlap spatially and temporally and hereby sets
the zero point of the clock. Furthermore, it becomes possible
to monitor the cross-correlation function between the shapes
of the pump and probe pulse. This measurement is repeated
for every probe wavelength, because the optical path length
Clusters of the desired charge state are collected by an
electrostatic lens and introduced into the first RF-quadrupole
(Q0). This ion guide has a large rod diameter of 20 mm and
consequently runs at a low frequency of 400 kHz. It is en-
closed in a gas cell flooded with He gas to a pressure of
10Ϫ2 mbar. Here, the clusters which are initially boiling hot
and occupy a large volume in real space and momentum
space are cooled and moderated by the friction they experi-
ence in the gas atmosphere. At the same time they experi-
ence the quadratic radial pseudopotential in the ion guide and
condense toward the central axis to form a beam of small
cross section. By this means the brightness of the source is
increased by about a factor of 50 compared to a direct cou-
pling of a mass-selective quadrupole ion guide to a sputter-
ing ion source of our kind. At the same time the temperature
of the clusters is reduced to about room temperature.12
Subsequently, the cluster ions of interest are selected
from the beam by a quadrupole mass filter (Q1). They are
introduced into a long homemade quadrupole ion guide (Q2)
which, for the purpose of the NeNePo experiments, serves as
an ion trap. This trapping mode of operation of a quadrupole
ion guide was first described by Dolnikowski and Watson.13
It employs the electrostatic entrance and exit lenses of the
ion guide and an integrated gas cell to confine the ions. The
entrance lens is fixed at a potential slightly below the initial
kinetic energy of the ions, while the exit lens is held at a
higher potential to reflect all ions in the trap. During their
first cycle in the trap, injected ions pass twice through the
gas cell where they lose kinetic energy by collisions with the
helium gas and cannot escape via the entrance lens. They are
therefore trapped inside the ion guide and oscillate with de-
creasing kinetic energy between entrance and exist lens.
Hence the ion trap can be filled with a continuous flow of
ions without ever pulsing the entrance or exit lens of the trap.
The accumulation only ceases when the ion density reaches
the space charge limit, which in our case is around
108 ions/cm3.
Inside the trap the anions interact with the laser light
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.70.241.163 On: Mon, 22 Dec 2014 08:56:54

J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Relaxation from linear to triangular Ag3
1019
ground state. We therefore used a wavelength of 370 nm as a
starting point of our femtosecond investigation.
For this experiment light pulses with a central wave-
length of 400 nm, a pulse duration of about 100 fs, and a
peak power density of 5•102 W/cm2 were used for photode-
tachment. Under these conditions, almost all Ag3 anions in
the path of the laser are neutralized. The pulses for photo-
ionization had a duration of 120 fs and a power density of
about 5•108 W/cm2. Cations emerging from the two-color
NeNePo process could be detected at probe pulse wave-
lengths between 385 and 345 nm whereas no cations were
detected at higher or lower photon energies. The apparent
width of this resonance can be attributed to the large differ-
ence between vertical and adiabatic electron affinity, which
was calculated to be 0.4 eV.7
FIG. 2. Agϩ3 yield from the NeNePo process as a function of the delay time
between photodetachment and photoionization at a wavelength of 415 nm.
At zero delay time between pump and probe pulse the
center of the resonance is found at 360 nm. It shifts to about
370 nm for delay times longer than 1 ps. In Fig. 2 the delay-
time-dependent cation yield is given for various wavelengths
of the ionizing radiation. The spectra are normalized to a
constant change in signal between ⌬tϭ0 and ⌬tϭ1 ps for
ready comparison in the time domain. Their overall shape
resembles our previous measurements. In all cases the signal
exhibits a minimum at ⌬tϭ0 and rises steeply to a maxi-
mum after about 500 fs. Then it converges to a constant
value at ⌬tϾ8 ps. Similar to the measurements with non-
resonant ionization, in no case is any oscillatory behavior
found. As wavelengths go from longer to shorter, the initially
rising slopes of the spectra move gradually to shorter times.
The signal reaches half its maximum amplitude after 300 fs
in the case of ionization at 384 nm but after only 110 fs in
the case of 347 nm. This behavior corresponds to a time-
dependent absorption spectrum of the molecule ͑compare
discussion below͒.
A photon energy of 3 eV for the photodetachment is well
above the calculated and measured vertical detachment en-
ergy ͑VDE͒ of AgϪ3 ͑2.44 eV͒. In order to clarify the possible
influence of this excess energy we performed a second set of
two color pump and probe measurements where one of the
light pulses was tuned to 495 nm in order to fit exactly to the
VDE of the anion. The second pulse remained at a wave-
length of 400 nm. Applying the light of only one of the
pulses, the anion signal due to photodetachment showed the
same depletion in either pulse, whereas the cation signal re-
sulting from photodetachment and photoionization from the
same pulse was ten times higher for 495 than for 400 nm.
This strong enhancement of the cation signal was found only
in a narrow wavelength region between 495 and 500 nm. A
typical pump and probe transient using both colors is given
in Fig. 3. Positive delay times denote that the pulse at around
500 nm comes first, serving as pump pulse, and the blue
pulse at 400 nm is then used for photoionization. For nega-
tive delay times the function of pump and probe pulse is
reversed accordingly. In contrast to the previously discussed
measurements, we now observe a strong maximum in cation
intensity at ⌬tϭ0. The overall delay-time-dependent signal
can be decomposed into the peak at time zero when both
pulses coincide and a shoulder at ⌬tϭ500 fs, as indicated in
Fig. 3. The straight line shows the difference between the
through the optical parametric generator changes slightly
with the wavelength setting.
RESULTS
The silver trimer anion is a linear molecule, while the
neutral forms an obtuse triangle. It is therefore a model sys-
tem for a large amplitude rearrangement of a molecule which
starts from a saddle point of the potential energy surface and
involves the breaking of the initial Dϱh symmetry.
In earlier experiments1 we used pump and probe pulses
of the same wavelength around 400 nm for photodetachment
and ionization. A typical measurement is reproduced in Fig.
2 for reference. It gives the yield of silver trimer cations as a
function of the delay time between photodetachment and
photoionization. Here we observed no sign of a periodic
bending motion of the neutral molecule which would be re-
flected in an oscillating cation yield. Instead the signal shows
a pronounced minimum at zero delay time, a single maxi-
mum around ⌬tϭ500 fs, and then converges to a constant
value after a time of about 2 ps. This behavior was inter-
preted as a consequence of ͑a͒ the time-dependent vertical
ionization potential of the molecule and ͑b͒ a fast conversion
of kinetic energy from the initial bending motion into the
two remaining vibrational degrees of freedom. The first ar-
gument accounts for the minimum at the delay time ⌬tϭ0
and the rise in signal to a maximum at ⌬tХ500 fs. The sec-
ond argument explains why there is no oscillatory behavior
visible at delay times ⌬tϾ1 ps. At that time it was not clear
whether this was because of ultrafast loss of the molecular
coherence within less than one period of the bending vibra-
tion or because the motion remained coherent but the coher-
ence could not be detected due to the nonresonant character
of the probe process.
In order to clarify this question we performed new
experiments employing resonant two-photon ionization of
the neutral silver trimers. Fortunately, a state which lies
energetically above half the adiabatic ionization potential
of the molecule is known from continuous wave ͑cw͒
spectroscopy.14,15 It has been tentatively assigned to 2E
Љ
symmetry and its 0-0 transition lies 26 971 cmϪ1 above the
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.70.241.163 On: Mon, 22 Dec 2014 08:56:54

1020
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Leisner et al.
FIG. 3. NeNePo signal of Agϩ3 obtained at a photodetachment wavelength
of 500 nm, and a photoionization wavelength of 400 nm ͑positive delay
times͒.
measured signal ͑hollow circles͒ and a fit to the peak at ⌬t
ϭ0, but is shifted vertically for clarification. The resulting
transient clearly resembles the time-dependent signal in the
one-color experiment mentioned above ͑refer to Fig. 2͒.
DISCUSSION
For the set of measurements using the 2E state as inter-
Љ
mediate resonance ͑Fig. 4͒, we find a minimum in the ion
signal for delay time ⌬tϭ0. This behavior is similar to the
one-color experiments published previously ͑compare Fig.
2͒, but for a different reason. In the new experiments we are
in a wavelength regime where the energy is sufficient to
ionize in a two-photon process, even at time zero. Hence, the
modulation of the ion signal with time is due to the change
FIG. 4. NeNePo signals of Agϩ3 obtained using resonant two-photon ioniza-
tion at various wavelengths.
the system after the collision depends strongly on the initial
temperature of the anionic cluster ensemble. At an initial
temperature of 50 K, the trajectories stay bundled in space
immediately after the collision and the system starts to oscil-
late predominantly along the stretching coordinate, exhibit-
ing resonant IVR. At 300 K the trajectories diverge substan-
tially and both bending and symmetric stretching vibrations
are excited, giving rise to dissipative IVR.
of potential energy difference between the ground state and
2
the E state of the neutral Ag along its coordination path
Љ
3
for relaxation. Nevertheless, the spectra show no oscillatory
motion of the Ag3 molecules after the first relaxation. We
therefore assume that under our experimental conditions the
coherence of the molecular ensemble is destroyed within the
first vibrational period.
This conclusion is supported by recent calculations of
Bonacic-Koutecky and colleagues,7 who modeled the
NeNePo investigations of Ag3. Among other calculations,
they followed bunches of classical trajectories on the ab ini-
tio potential energy surface of the neutral trimer belonging to
a sample of initial conditions at various temperatures of the
anionic clusters. Their results confirm the picture of the mo-
lecular motion after photodetachment as presented in Ref. 2:
The neutral molecule is initially linear and then bends with
increasing acceleration to reach the triangular configuration.
The initial bending motion follows a narrow valley in the
potential energy surface and involves at the same time a
slight stretching and subsequent recontraction of the mol-
ecule. After a certain time, which depends strongly on the
initial temperature of the cluster ensemble, the bending mo-
tion of the molecule comes to an abrupt standstill at the
turning point when the terminal atoms collide. According to
these calculations, the amount of coherence that remains in
The molecular motion is best rationalized in a one-
dimensional cut through the potential energy surfaces ͑PES͒
along the bending mode Qx , ͑see Fig. 5͒. The calculated
potentials for the three charge ground states are shown in
Fig. 5 by solid lines. The dashed lines give a qualitative
picture of the resonant states involved in this ionization pro-
cess and are taken from preliminary calculations. After pho-
todetachment from the anion ͑bottom slim arrow͒ a molecu-
lar wave packet is formed in the linear configuration of the
neutral molecule ͑at Qxϭ5.2 au͒. Though this configuration
appears to be a local minimum of the neutral PES, there is a
monotonically decreasing pathway toward the triangular
configurations around Qxϭ0 which involves a change in the
symmetric stretching coordinate. Along this path the mol-
ecule bends, so that after a delay time of 500 fs, resonant
2
ionization via the E state of the neutral Ag becomes pos-
Љ
3
sible in a large Franck–Condon regime ͑broad arrows͒ due to
the similar slopes in the PES. This resonant ionization path-
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.70.241.163 On: Mon, 22 Dec 2014 08:56:54

J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Relaxation from linear to triangular Ag3
1021
time zero. This can only be explained if a second resonant
state is involved in the ionization process. Such a state is
indeed predicted by the calculation, and in Fig. 5 the two
2
2
proposed participating states are labeled A1 and B1 , re-
spectively. They have opposite slopes along the bending
mode Qx so that only at ⌬tϭ0 the two transition energies
coincide accidentally.
CONCLUSION
We presented extended measurements on the Ag3 mol-
ecule using a bound–bound transition via intermediate reso-
nant states. Two excited states are seen at short delays as a
resonance in the linear geometry of the molecule. The strong
change of the ion signal close to zero delay time implies
large differences in the gradients of the potential energy sur-
faces involved in the ionization process, particularly in the
region near the linear configuration of the molecule. The 2E
Љ
excited state we believe to contribute to the resonant absorp-
tion after partial relaxation of the linear molecule. Here, the
maximum in ionization probability was found in the geom-
etry of an obtuse isosceles, nearly equilateral triangle. The
lack of oscillation we attribute to fast IVR processes after
collision of terminal atoms along the relaxation path. Over-
all, both the theoretical and the experimental findings reflect
the tendency of collisions to enhance the spread in phase
space of partially coherent systems and provide a molecular
understanding of the fast intermolecular redistribution pro-
cesses of vibrational energy that can be found in molecules
as small as a metal trimer.
FIG. 5. Cut along the bending coordinate Qx through the potential energy
surfaces of the resonant states of Ag3 used for the NeNePo experiments. The
optical transitions are indicated as arrows.
way gives rise to the spectra given in Fig. 4. As a conse-
quence of the IVR processes discussed above, this part of the
PES is not revisited at later times and therefore the signal in
Fig. 4 declines at larger delay times.
ACKNOWLEDGMENTS
´
We thank the group of Professor Bonacic-Koutecky for
fruitful discussions. This work was supported by the Deut-
sche Forschungsgemeinschaft in the Sonderforschungsbere-
ich 337.
Tuning the wavelength for one pulse from the UV ͑370
nm͒ into the green ͑500 nm͒ as done in the second set of
measurements ͑Fig. 3͒, the picture changes completely. In-
stead of a minimum we find a maximum in cation intensity
for delay time zero. Additionally, a shoulder is found in the
cation signal at positive delay times around ⌬tϭ500 fs. As
can be seen from the decomposition of the data performed in
Fig. 3, the shoulder clearly resembles the signal obtained in
the one-color experiments at 400 nm ͑cf. Fig. 2͒. As in both
cases the ionization is performed at 400 nm, we conclude
that a variation of the photodetachment wavelength from 400
to 500 nm has no marked effect on the subsequent bending
dynamics of the neutral molecule. This implies that most of
the excess energy of the photodetachment laser is transferred
into the photoelectron and does not remain in the molecule.
The most prominent feature in the spectrum, however, is the
peak around ⌬tϭ0. For energetic reasons, such an ionization
requires the absorption of three photons. Because of the
strong enhancement in the ion signal using the pulse at 500
nm compared to the one at 400 nm, we assume that the linear
silver trimer can be ionized resonantly at 500 nm. If there
was only one resonant state participating in the process, one
should expect to observe a wavelength shift of the resonance
as the wave packet evolves on the PES. Instead, the signal
vanishes at all delay times outside a narrow range around
1
¨
S. Wolf, G. Sommerer, S. Rutz, E. Schreiber, T. Leisner, L. Woste, and R.
S. Berry, Phys. Rev. Lett. 74, 4177 ͑1995͒.
2 T. Leisner, S. Rutz, G. Sommerer, S. Vajda, S. Wolf, E. Schreiber, and L.
¨
Woste, in Fast Elementary Processes in Chemical and Biological Systems,
AIP Conf. Proc. No. 364 ͑AIP, New York, 1996͒, p. 603.
3 R. S. Berry, V. Bonacic-Koutecky, J. Gaus, T. Leisner, J. Manz, B.
Reischl-Lenz, H. Ruppe, S. Rutz, E. Schreiber, S. Vajda, R. de Vivie-
¨
Riedle, S. Wolf, and L. Woste, Adv. Chem. Phys. 101, 101 ͑1997͒.
4 D. W. Boo, Y. Ozaki, L. H. Andersen, and W. C. Lineberger, J. Phys.
Chem. A 101, 6688 ͑1997͒.
5 H. O. Jeschke, M. E. Garcia, and K. H. Bennemann, Phys. Rev. A 54,
R4601 ͑1996͒.
6 H. O. Jeschke, M. E. Garcia, and K. H. Bennemann, J. Phys. B 29, L545
͑1996͒.
7
´
M. Hartmann, J. Pittner, V. Bonacic-Koutecky, A. Heidenreich, and J.
Jortner, J. Chem. Phys. 108, 3096 ͑1998͒.
8
´
M. Warken and V. Bonacic-Koutecky, Chem. Phys. Lett. 272, 284 ͑1997͒.
9 O. Rubner, C. Meier, and V. Engel, J. Chem. Phys. 107, 1066 ͑1997͒.
10 C. Yannouleas and U. Landman, J. Phys. Chem. A 102, 2505 ͑1998͒.
11
¨
¨
¨
R. Keller, F. Nohmeier, P. Spadtke, and M. H. Schonenberg, Vacuum 34,
31 ͑1984͒.
12 L. Hanley, S. A. Ruatta, and S. L. Andersson, J. Chem. Phys. 87, 260
͑1987͒.
13 G. G. Dolnikowski, M. J. Kristo, C. G. Enke, and J. T. Watson, Int. J.
Mass Spectrom. Ion Phys. 82, 1 ͑1988͒.
14 K. LaiHing, P. Y. Cheng, and M. A. Duncan, Z. Phys. D 13, 161 ͑1989͒.
15 E. E. Wedum, E. R. Grant, P. Y. Cheng, K. F. Willey, and M. A. Duncan,
J. Chem. Phys. 100, 6312 ͑1994͒.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.70.241.163 On: Mon, 22 Dec 2014 08:56:54