ZEKE spectroscopy of the AgNH3 complex

Jun Miyawaki a,*, Dong-Sheng Yang b, Ko-ichi Sugawara a

a Nanotechnology Research Institute, AIST, Nanocluster Group, Central 5, Higashi 1-1, Tsukuba, Ibaraki 305-8565, Japanb Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055, USA

Received 20 December 2002; in final form 27 January 2003

Abstract

The vibrational structure of the AgþNH3 complex has been investigated by single-photon zero kinetic energy

(ZEKE) photoelectron spectroscopy. The intermolecular stretching mode with a frequency of 375 cm�1 is measured

from the ZEKE spectrum, whereas the intermolecular bending vibration is not observed, in contrast to the~AA2E1=2–~XX

2A1 electronic spectrum of the neutral AgNH3 complex [Miyawaki et al., Chem. Phys. Lett. 302 (1999) 354].

The ionization potential of AgNH3 was determined to be 47580 cm�1 (5.899 eV).

� 2003 Elsevier Science B.V. All rights reserved.

1. Introduction

In the past two decades, experimental and the-

oretical studies of metal–molecule complexes have

been devoted to understanding the metal–ligand

bonding in the electronic states of neutral, cat-

ionic, and anionic systems. Experimentally, a va-

riety of spectroscopic techniques have been applied

to metal atom or ion–molecule complexes to ob-tain their structural and dynamic information (see,

e.g., [1]).

At Tsukuba, we have been studying the excited

electronic states of the neutral silver–ammonia

complexes [2–4] and have reported the resonant

two-photon ionization (R2PI) spectrum of the~AA2E1=2;3=2–~XX

2A1 transition of the AgNH3 complex

[2,3]. The origin of the ~AA–~XX band system of

AgNH3 is red-shifted by more than 1 eV from thecorresponding atomic Ag(5p2P–5s2S) transition,

indicating a large stabilization of the ~AA state.

Calculations by Archirel et al. have showed that

this stabilization results from the �ionic� nature ofthe ~AA state, with a strong interaction between NH3and the Agþ core that has a �spectator� electron inthe 5pp orbital [5]. The R2PI spectrum exhibits a

fairly clear vibrational structure, however, its in-terpretation has not been straightforward. The

intermolecular bending mode in the ~AA state has amuch smaller frequency (185 cm�1) than the value

predicted by Archirel et al. (574 cm�1) [5], and is

observed only in the 2E1=2–2A1 band system. We

have rationalized this anomaly as the result of the

Jahn–Teller and spin–orbit interactions in the ~AA2Estate [3].In this Letter, we report the adiabatic ionization

potential (IP) of AgNH3 and the vibrational

structure of AgþNH3, obtained from its single-

photon zero kinetic energy (ZEKE) photoelectron

Chemical Physics Letters 372 (2003) 627–631

www.elsevier.com/locate/cplett

* Corresponding author. Fax: +81-298-61-4804.

E-mail address: j.miyawaki@aist.go.jp (J. Miyawaki).

0009-2614/03/$ - see front matter � 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0009-2614(03)00469-X

spectrum, and compare the vibrational structures

in the ionic ground state with the neutral excited ~AAstate. The silver ion has a closed electronic-shell of

4d10, and is expected to form the AgþNH3 complex

in the ground electronic term of 1A1. Therefore,

the vibrational structure of AgþNH3 should besimpler than that of the ~AA2E state of AgNH3 andprobably easier to understand. This study is also

one of a series of ZEKE studies on metal–ammo-

nia (1:1) complexes, which began from our first

report on AlNH3 [6], followed by InNH3 [7] and

GaNH3 [8] in D.S.Y�s laboratory. ZEKE spectraof similar metal–molecule complexes have also

been reported by other groups [9,10].The ionization potential of AgNH3 and the vi-

brational frequencies of AgþNH3 have been stud-

ied by photoionization spectroscopy and ab initio

calculations [5,11–13]. Radloff and co-workers

measured the photoionization efficiency (PIE)

spectra of AgðNH3Þn ðn ¼ 1–20Þ and determinedtheir IPs to examine the size-dependent properties

of these clusters [11,12]. The PIE curve of AgNH3rises slowly without a steep ionization threshold.

Nonetheless, they were able to determine the IP of

AgNH3 to be 5:80� 0:02 eV from a weak but

significant first step in the spectrum [12,13]. Ritze

and Radloff performed ab initio calculations for

the electronic ground states of the neutral and

ionic AgNH3 complex to compare them with the

experiment [13]. The calculated IP ¼ 46935 cm�1

(5.819 eV) well reproduced the measured value.

They also calculated the vibrational frequencies of

the complex in the neutral and ionic states and

obtained the intermolecular stretching frequency

of 370 cm�1 for AgþNH3. From this frequency

and the Franck–Condon calculation, they simu-

lated the PIE spectrum. However, an accurate

comparison of the simulated and experimentalspectra was not possible, due to the lack of clear

steps in the observed spectrum.

In the PIE spectrum of AgNH3 obtained from

the present study, a sharp stepwise structure is

observed, which allows unambiguous identifica-

tions of the ionization onset of AgNH3 and the

inception of the vibrational excitation of AgþNH3.

The clearer stepwise structure in our PIE spectrumis probably due to an efficient cooling of the cluster

beam in our experiment. In the ZEKE spectrum,

very sharp vibrational bands are observed, which

enable us to accurately determine the IP of the

neutral complex and the intermolecular stretching

frequency of the corresponding ion, and to com-

pare these values with the theoretical calculations

[5,13].

2. Experimental method

The experiments were carried out at Tsukuba.

The AgNH3 complex was synthesized by the re-

action of the laser-vaporized silver atom with

ammonia. The atomic silver was produced by fo-cusing the second harmonic output (532 nm) of a

Nd:YAG laser (Comtinuum Surelite I) on the

surface of a silver rod. The ammonia gas (Tak-

achiho, 99%) was seeded typically 2–5% in He gas

and was expanded through a pulsed nozzle (Gen-

eral Valve 9-279-900) into a reaction channel

(2 mm diameter, 20 mm length). The silver–am-

monia complexes were produced in the reactionchannel and cooled by supersonic expansion into

the vacuum. The complexes were introduced

through a skimmer (2 mm) into a detection

chamber and then irradiated by a UV laser. The

skimmer was located 40 mm downstream from the

exit of the reaction channel, whereas the laser in-

terception with the complex occurred at 170 mm

from the skimmer.The UV laser was a frequency-doubled dye la-

ser (Lambda Physik SCANmate) pumped by a

XeCl excimer laser (Lambda Physik COMPex

100), which was used for both the photoionization

and ZEKE measurements. The linewidth of the

dye laser was �0.15 cm�1 and its wavelength was

calibrated against the absorption spectrum of130Te2 [14]. Ionized complexes were extracted andmass-selected by a Wiley–McLaren type time-of-

flight (TOF) mass spectrometer and detected by a

microchannel plate detector (Hamamatsu F4655).

The PIE spectrum was observed as the AgþNH3signal intensity of a function of the laser wave-

length.

Prior to the ZEKE measurements, the experi-

mental conditions, such as sample gas concentra-tion and stagnation pressure, power of the

ablation laser, timing of the gas pulse, the ablation

628 J. Miyawaki et al. / Chemical Physics Letters 372 (2003) 627–631

and excitation lasers, were optimized to maximize

the AgþNH3 signal in the TOF mass spectrum. In

the ZEKE experiment, the complex was first ex-

cited to high-lying Rydberg states and then ionized

by a time-delayed voltage pulse applied to the re-

peller. Typically, a field of 2.0 V/cm was applied bya digital delay generator (Stanford, DG535) for

100 ns with a 3 ls delay from the excitation laser.The ZEKE electrons produced from the delayed

field ionization were extracted by the same electric

pulse in the direction opposite to the TOF mass

spectrometer and detected by a channeltron (Mu-

rata Ceratron). A small spoiling DC field (0.2 V/

cm) was applied to remove the prompt electronsfrom the photoionization, and a l-metal cylinderwas used to encase the ionization region to shield

the ZEKE electrons from the external magnetic

field that otherwise would reduce the electrons.

The ZEKE signal capacitatively decoupled from

the anode was amplified by a preamplifier (NF

BX-31A), averaged by a gated boxcar (Stanford,

SR250) and stored in a personal computer throughan A/D convertor.

3. Results and discussion

The single-photon PIE spectrum of AgNH3 in

the region of 47050–48430 cm�1 is shown in Fig. 1.

The spectrum exhibits a clear onset of theAgþNH3 signal at 47520 cm

�1 as indicated by the

arrow. The IP of AgNH3 is estimated to be

�47577 cm�1, after a þ57 cm�1 correction for the

energy shift induced by the electric field (87 V/cm)

applied in the extraction region. In addition, the

spectrum displays two other large steps at �47890and �48260 cm�1. These steps with a �370 cm�1

interval indicate the opening of excited channels ofthe intermolecular stretching mode in AgþNH3 as

is seen in other M–NH3 (M ¼ Al, In, Ga) com-plexes [6–8]. This stepwise structure is the one that

Ritze and Radloff expected from their calculations

[13]. Their simulation excellently reproduces the

spacing of the steps and intensity pattern observed

in our PIE spectrum, except for the small differ-

ence in the onset of ionization.The PIE spectrum also shows a number of

sharp peaks superimposed on the stepwise struc-

ture. These peaks are from resonant transitions to

the Rydberg states converging to the higher vi-

brational levels in the ionic state, which are auto-

ionized to ionization continua. This kind ofautoionization structure is often seen in the PIE

spectra of molecules and may sometimes be used

to determine the ionization thresholds of excited

vibrational levels in an ionic state (see, e.g., [15]).

However, the number of these peaks observed in

the PIE spectrum is too small to make such an

analysis.

Fig. 2 shows the single-photon ZEKE spectrumof AgNH3 in the region of 47300–48400 cm

�1. The

ZEKE spectrum consists of the intermolecular

stretching (m3) progression, as expected from the

stepwise structure in the PIE spectrum. The peak

at 47576 cm�1 is assigned to the origin band, in

accordance with the PIE spectrum. The spectral

linewidth (FWHM) of this band is �10 cm�1,

which includes unresolved rational lines and theeffect of the electric field used for ionization.

To determine a more accurate IP from the

ZEKE origin band, measurements of the field-

strength dependence of the band positions are

necessary. In the present experiment, however,

Fig. 1. The single-photon PIE spectrum of AgNH3. The onset

of the ionization signal located at 47520 cm�1 is indicated by an

arrow. The IP of AgNH3 is estimated to be �47577 cm�1 by

taking into account the shift of 57 cm�1 caused by the electric

field (87 V/cm). The stepwise increase in the signal with a

spacing of �370 cm�1 indicates the excitation of the intermo-

lecular stretching (m3) mode in the ionic state.

J. Miyawaki et al. / Chemical Physics Letters 372 (2003) 627–631 629

such systematic measurements were not possible

because of the limited size of the ZEKE signal.

Alternatively, we refer to the IP of the Al atom,

which lies in this region. Under the same experi-

mental conditions, the ZEKE peak of Al from the

ground 3p2P1=2 state is observed at 48275 cm�1

with a linewidth (FWHM) of �5 cm�1. This wavenumber is �4 cm�1 smaller than the accurately

measured IP of Al, 48278:480� 0:003 cm�1 [16].

Thus, we estimate that the field effect in the present

experiment lowers the band position from the true

IP by �4 cm�1. Therefore, the IP of AgNH3 is

determined to be 47580� 5 cm�1 by taking the

field effect and the bandwidth into account. This

value (5.899 eV) is in good agreement with thecalculated values of 5.92 eV [5] and 5.82 eV [13].

The IP of AgNH3 is 1.67 eV smaller than that of

atomic silver (7.566 eV [17]), which indicates the

significant increase in the binding energy of the

complex upon ionization. It is understood from

the different bonding schemes in the two states that

in the neutral ground state, the complex is domi-

nantly bound by the dipole–(induced dipole) in-teraction, whereas the charge–dipole interaction

significantly stabilizes AgþNH3.

The additional bands at 47946 and 48311 cm�1

are assigned to the 310 and 320 bands, respectively.

Fitting all the ZEKE peak positions to

T ðvþ3 Þ ¼ Tþe þ xþ

3 ðvþ3 þ 1=2Þ � xþ3 x

þ3 ðvþ3 þ 1=2Þ2

gives a harmonic frequency of 375 cm�1 and an

anharmonicity of 2:5 cm�1. The observed fre-

quency is in good agreement with the calculated

values of 370 cm�1 [13] and 388 cm�1 [5], and is

only slightly larger than the frequency of the samemode experimentally observed for the ~AA2E state ofthe neutral AgNH3 (371 cm

�1) [3]. This result

supports the fact that the ~AA state has an �ionic�nature, as described by Archirel et al.

Although the ground state of AgþNH3 and the~AA2E state of AgNH3 share the ionic nature in themetal–ligand bonding, as indicated by the similar

frequency for the intermolecular stretching mode,the ZEKE spectrum of AgNH3 is significantly

different from its R2PI spectrum for the intermo-

lecular bending (m6) mode. The ZEKE spectrumexhibits no transition involving the m6 mode, whosefrequency is predicted to be 632 and 658 cm�1, by

Archirel et al. and Ritze and Radloff, respectively.

This very small Franck–Condon factor for this

mode is in accordance with the prediction by Ritzeand Radloff. In contrast, the R2PI spectrum of

AgNH3 shows a strong Franck–Condon activity

for this mode, with the 610 band intensity being

about one half of the 310 band intensity, but only in

the 2E1=2–2A1 band system. Furthermore, its fre-

quency (185 cm�1) is significantly smaller than the

theoretically predicted value, 574 cm�1, that is

comparable to the m6 frequency of AgþNH3 [5].

This difference between the ZEKE and R2PI

spectra again highlights an anomaly of the excited~AA state in the neutral AgNH3 complex. The

~AA2E1=2state of AgNH3 must have a very different poten-

tial energy surface shape from that of the ground

state of AgþNH3 along the m6 coordinate, proba-bly due to the Jahn–Teller and spin–orbit inter-

actions in the ~AA2E state.In the R2PI spectrum of AgNH3, a small peak

was observed at 175 cm�1 below the energy of the~AA2E1=2–~XX

2A1 origin band when the cooling of the

cluster beam is less efficient. This band was at-

tributed to the hot band from the v3 ¼ 1 level inthe ground state. In the ZEKE spectrum, such hot

bands are not observed beyond the signal-to-noise

ratio for any of the vþ3 ¼ 0–2 bands, indicating thatthe vibrational temperature is extremely low. At-

tempts to make warmer cluster beams by changing

Fig. 2. The single-photon ZEKE spectrum of AgNH3. Vibra-

tional mode mþ3 denotes intermolecular stretching.

630 J. Miyawaki et al. / Chemical Physics Letters 372 (2003) 627–631

the experimental conditions (higher ammonia

concentration, lower stagnation pressure, etc.) led

to an increase of other ion signals or a decrease in

the AgþNH3 intensity, which just deteriorated the

ZEKE spectrum of AgNH3.

The IP of AgNH3 determined in this study,5.899 eV, is as much as 0.1 eV higher than the

value, 5:80� 0:02 eV, observed by Radloff and co-workers from their PIE spectrum. One of possible

reasons for this discrepancy may be due to the

different electric field-strengths applied to the ex-

traction region in the two experiments. However,

the IP difference of 0.1 eV is too large to be ac-

counted for only by this effect, because the shift isonly, for example, 0.025 eV for a field of 1000 V/

cm. Another possible cause may arise from the

difference in the cluster beam temperatures, which

can lead to thermal populations of the neutral

complex at excited vibrational levels. The PIE

spectrum observed in our experiment is free from

the hot band contribution as confirmed from the

ZEKE spectrum. On the other hand, Radloff andco-workers estimated the vibrational temperature

of the AgðNH3Þn clusters in their experiment to belower than 120 K, by measuring the relative

strength of the hot bands of the Ag2 dimer [12]. At

the vibrational temperature of 120 K, the lowest

excited v3 ¼ 1 ðEv ¼ 175 cm�1Þ state of AgNH3populates 12% of the vibrational ground state, and

may lower the onset of the AgþNH3 signal in thePIE spectrum. Moreover, transitions from this

state also appear almost halfway between the steps

observed in the cold PIE spectrum, and thus may

make their PIE curve less-structured. Although the

temperature difference is obviously responsible for

the discrepancy, the temperature of 120 K is not

enough to fully account for it. It is possible that

the vibrational temperature of AgNH3 in theirexperiment was much higher than what was esti-

mated, or that their PIE spectrum suffered from

some other factors, such as two-photon ionization

and fragmentation from larger clusters.

Finally, the ZEKE spectrum of AgNH3 exhibits

an intensity increase from the origin band to the 320band, indicating that there are more ZEKE bands

at higher wave numbers. However, in our currentsetup, the shortest laser wavelength is limited to

�206 nm by the non-linear b-barium borate

crystal used for the second harmonic generation.

Alternatively, one can perform a resonant two-

photon ZEKE experiment through the excited ~AAstate as an intermediate to investigate the higher

energy region of AgþNH3. This two-photon ex-periment also has the advantage that different

Franck–Condon regions are accessible by varying

the intermediate vibronic levels. Therefore, the

intermolecular bending (m6) mode of AgþNH3 that

is completely dark from the neutral ground state,

may be observed in the two-photon ZEKE spectra

by exciting the v6 levels in the ~AA state. Such work isnow in progress in our laboratory.

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