Spectroscopy of jet-cooled Ag2Au

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CHEMICAL PHYSICS LETTERS

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Spectroscopy of jet-cooled Ag,Au Jacqueline C. Pinegar, Jon D. Langenberg and Michael D. Morse Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA

Received 12 July 1993

Resonant two-photon ionization spectroscopy has been usedto study the supersonically cooled metal cluster, A&u. One electronic band system has been observed, with the origin band at I7525 cm-‘. Vibronic progressions have been found and analyzed using a least-squares fitting procedure. The molecule is assigned as belonging to the C& point group in both the ground and the excited electronic states, and vibrational progressions in both of the two totally symmetric (a,) vibrational modes have been identified, giving values of w’, =200.15f0.49 cm-’ and oi=111.28f0.62 cm-’ for the excited electronic state of the io7Ag“‘sAg’97A~ isotopic modification. Anharmonicities x; , , &, and x;, are also reported for the excited electmnic state, along with lifetime measurements for several vibronic levels. This work also places the ionization potential of Ag,Au in the range 5.006 IP(Ag,Au) < 6.42 eV.

Although the electronic structure of metallic atoms can be calculated fairly accurately using the methods of ab initio quantum chemistry and infinite metallic solids are rather well understood through the methods of solid state physics, the intermediate size range corresponding to finite metal clusters is understood to a much lesser extent. A major aim of current spectroscopic research is to characterize these small metal clusters spectroscopically, thereby providing fundamental information about the electronic structure and chemical bonding in metals. As a small contribution towards this effort, we present and analyze the spectrum of an electronic band system of the triatomic metal cluster, Ag,Au, in this Letter. Detailed spectroscopic studies of gas-phase metal clusters, in which vibrational structure is resolved and analyzed, have been limited almost exclusively to metal dimers and trimers. One metal tetramer (Cut ) has been investigated at the level of vibrational resolution [ 11, while only a few metal triatomics have been successfully investigated at this level. To our knowledge, the only examples are the alkali clusters Li, [ 21, Na, [3-l 21 and LiXNaB_X [ 13,141, the coinage metal (pseudoalkali) clusters Cu, [ 15-181, Ag, [ 191, Cu,Ag [20], CuZAu [20] 458

metals Als [ 2 1 ] and Big [ 221, and the transition metal cluster Nij [ 231. Of these species, the alkali and coinage metal trimers have yielded the greatest number of successful investigations, primarily because they possess only one valence electron per atom, making for a relatively simple electronic structure. In addition, the relatively low boiling points of the alkali metals allow higher pressures of these elements to be generated readily, also contributing to the greater success in studying them. The homonuclear alkali and coinage metal trimers are all expected to possess ground electronic states of g 2E’ in the D3,, point group, deriving from the molecular orbital configuration a’,*e”. The 8 ‘E’ state is orbitally degenerate, however, and is therefore subject to a Jahn-Teller distortion, which lowers the symmetry to C2”. This breaks the degeneracy of the 2 ‘E’ state (in D3,,) into 2A, and 2B2 components (in C,,). For heteronuclear species, such as CuzAg [ 201, CuzAu [20] and in this work, Ag2Au, the expected symmetry is CZvr and the ground states will belong to either the ‘Ai or the ‘B2 symmetry species. and CuAgAu [ 201, the p-block

1. Introduction

0009-2614/93/% 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

Volume 2 12, number 5

CHEMICAL PHYSICS LETTERS

17 September 1993

‘07Ag’09Ag’97Au in the energy region of 17450 to 18900 cm-‘, recorded using rhodamine 590, fluorescein 548, and coumarin 540A dye laser radiation. The spectrum is quite clean, with progressions in two modes easily assignable. In CZv,the AgzAu molecule will have two modes of a, symmetry, and one mode of b2 symmetry. If the homonuclear trimers are used as a model, then a relatively high-frequency breathing mode (of a1 symmetry), and two bending modes of similar frequency (of a, and b2 symmetries) are expected. Since electronically allowed excitations from the ground vibrational level of the R state will have nonvanishing Frank-Condon factors only if they terminate on vibrational levels of A, symmetry in the excited electronic state, only even quanta excitations of the b2 mode should be observed. Further, these should be weak compared to excitations of the al modes, so it is not surprising that they are not observed. Identifying the higher frequency breathing mode as mode 1, and the a, bending mode as mode 2, progressions involving both modes are readily observed and assigned. These are identified in fig. 1 using the notation l&2’,to indicate a transition originating from the ground vibrational level which simultaneous excites vibrational mode 1 with i quanta and vibrational mode 2 with j quanta. Vibronic band positions for the ‘07Ag’09Ag’97Au (mass 4 13) isotopic modification are listed in table 1, along with isotope shifts for the mass 411 ( lo7AgZ19’Au) and mass 415 (lo9Agz19’Au) fea-

2. Experimental

In this study, the Ag2Aumolecules were formed by pulsed laser vaporization (Nd : YAG, 532 nm, 15-20 mJ/mm* ) of a 2 : 1 (molar) alloy of Ag and Au, followed by supersonic expansion in helium carrier gas. Experimental details have been described previously [ 201. The excitation laser, a Nd: YAG pumped tunable dye laser, was scanned from 16900 to 18890 cm- ‘. A fixed-frequency excimer laser (KrF, 248 nm, 5.00 eV) was used for photoionization of the excited molecules. To calibrate the spectrum, four bands were examined under high resolution ( RZ 0.04 cm- * ) while an absorption spectrum of gaseous I2 was recorded simultaneously. This was accomplished by narrowing the output of the dye laser by insertion of an airspaced intracavity &alon, which was then pressure scanned with CC&F2 (freon 12, DuPont). The I2 atlas of Gerstenkorn and Luc [ 241 was then used to provide absolute frequency calibration. However, because the Ag,Au signal levels were too low to be strongly observed in the high-resolution scan, the spectra were only calibrated to an accuracy of approximately 2 cm-‘.

3. Results Fig. 1 displays the low resolution ( N 0.5 cm-‘) resonant two-photon ionization spectrum of

Vibronic Spectrum of ‘mAg’OJAg’“Au

I

I7400

I

I

17600

I

1

17800

I

I

I

1Woo

I

18200

I

I

18400

I

I

18600

I

1

la800

I

19OLxl

Frequency (cm-‘)

Fig. 1. Resonant two-photon ionization spectrum of ‘07Ag1WAg’97A~, recorded using rhodamine 590, fluorescein 548, and coumarin 540A radiation in combination with KrF excimer radiation (248 nm) for photoionization.

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Table 1 Vibronic hands of to7AgiWAg’97A~ ‘~~1 Band

Measured frequency

Isotope shift (cm-‘) (cm-‘)

““Ags 19’Au

OX 2; I :;

‘)

c,

-0.65 (0.98) -0.44 (0.15) 0.45 (-1.02) 0.68 (0.22) 1.51 (-0.36) 0.87 (0.22) 1.42 (-0.51) 0.72(-1.09)

IWAg 19’Au 0.07 - 1.86 -0.72 -0.85 -0.70 -2.22 -1.32 - 1.39

(2.16) (-0.30) ( - 1.30) (0.30) (- 1.08) (-0.47) (-1.55) (-0.98)

17526.72 17636.41 17721.72 17746.04 17830.52 17856.48 17918.40 17939.73 17966.91 18026.95

( 1.68) (0.92) (- 1.41) b*c) (0.09) (- 1.41) (0.09) (- 1.58) (- 1.01) Q) (0.07) (-0.18)

0.46 (-0.59)

- 1.40 (-0.06)

18049.70 18076.63 18115.42 18134.88 18158.35 18221.72 18241.78 18310.76 18327.01 18349.45

(0.17) ( -0.66) (-0.16) b, (0.60) (0.02) (0.63) (0.35) (0.81) (0.42) b=) (0.87)

1.30 1.07 1.08 1.44 1.56 1.12 1.63 2.35 1.97 1.65

(0.36) (-0.57) (-0.01) (0.70) (0.21) (0.19) (0.22) (1.37) (0.24) (0.38)

-2.20 - 3.07 - 1.88 -0.42 - 1.80 -2.61 - 2.78 - 2.52 -2.55 -4.39

18413.75 18432.27 18504.80 18517.75 18537.35 18604.45 18621.47 18707.40 18794.50

(-0.06) (0.18) (1.73) (0.09) (-0.24) (-0.83) (-0.04) ‘) (-0.09) (- 1.02)

2.33 (-0.27) 2.80 (0.28)

- 2.54 - 2.93 -4.34 -2.90 -3.79 -4.05 -3.67

(0.20) (0.22) (0.88) (0.30) (-0.34) (- 1.29) (-0.10)

-3.80

(-0.33)

.

2.91 3.50 4.12 3.55

(-0.25) (0.06) (-0.52) (-0.41)

(-0.03)

( -0.27) (-0.08) (2.06) (0.99) (0.13) (0.00) (0.97) (0.31) (-0.34)

‘r Vibronic bands were tit to the formula v=T,+Z, [c&u; t~~~(u~+~~)]+~~~~~;[u~u~ t j(v; tu;)]. The resulting values of To, o;, wi, x’,1,xiz. and x’,s arc given in table 2, along with their 1u error limits. ” Lifetimes of the 100, 120, 300 and 320 levels were measured by exciting the 1A, lb2& l& and l&2; bands using the time-delayed resonant two-photonmethod [21]. The resulting values are: r(100)=511+34ns, 7(120)=457+28 ns, 7(300)=658+ 128 ns, and

r( 320) = 598 k 72 11s. ‘) Following each observed frequency, the residual Y,~- vdC is given in parentheses. d) Isotope shifts are given as v (isotope modification) - u( 107Ag’WAg’9’Au). Following each observed isotope shift, the residual v,,,,- Yap is given for the tit of that isotopic modification to the formula given in footnote a. The resulting values of the vibrational given in table 2, along with their 1u error limits. ‘) High-resolution scan of I2 was taken at this transition frequency to calibrate the spectrum.

tures. Vibrational constants obtained from fitting these band positions are listed in table 2. Excited state lifetimes are also given for the four excited state vibrational levels where this measurement was performed. In addition, the observation of the origin band at 17525 cm-’ using a KrF excimer laser for photoionization places the ionization potential for 460

constants are

AgzAu in the range of S.OO
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Table 2 Fitted vibrational constants for A&Au a’ Constant

i07Ag109*g!97*u

“‘Ag* ‘97A~

‘09Ag,r9’Au

TO

17525.04 (0.64) 200.15 (0.49) 111.28 (0.62) -0.62 (0.06) 0.00 (0.10) - 1.65 (0.09)

17525.09 (0.46) 199.82 (0.42) 111.53 (0.45) -0.48 (0.06) -0.02 (0.07) -1.53 (0.07)

17524.63 (0.70) 199.83 (0.55) I1 1.23 (0.69) -0.67 (0.07) -0.10 (0.10) -1.62 (0.11)

0; 0; Xi, x;2 Xi2

‘) All constants are reported in wavenumbers (cm-‘), followed by the Iu error limits in parentheses, Theseconstants were obtained by a least-squares tit of the data of table 1 to the expression v= T,tl& [CO@:t_&(v:* t u:) ] t Ii
tion is capable of one-photon ionizing the AgzAu molecules. This further limits the ionization potential to the range 5.00
ecules. In addition, however, the diffuse character of the silver 5s orbital causes Ag, to have a longer bond length, weaker bond strength, and lower force constant than either Cuz or AuZ, and it is likely that the diffuseness of this orbital also contributes to a weakening of the bonds in AgzAu as compared to CuzAu, CuzAg, or CuAgAu.

4. Discussion Acknowledgement Given the high energy of the first excited state of atomic silver [26] (4di05p’, 2P?,2, 29552.05 cm-‘) and the correspondingly high energies of the first known excited states of AgZ (v0,=22977.5 cm-‘) [27] and Ags (v,=27010& lOcm-‘) [ 191, it seems likely (but not definite) that the excitation involved in the observed spectrum of Ag,Au (voo= 17526.7 cm- ’ ) originates on the gold atom. The first excited state of atomic gold is 5d96s2, ZD5,2 at 9 16 1.3 cm-’ [ 261, with the *DJ,? excited spin-orbit component lying at 21435.3 cm-’ [ 261, so it is plausible that the observed band system of AgzAu involves the excitation of a 5d electron of gold into the o framework based on the 5s, and 6sAuatomic orbitals. High-level theoretical investigations will be necessary to verify (or disprove) this conjecture. The frequencies of the two a, vibrational modes determined for the excited state of Ag,Au (200 and 111 cm-l ) are substantially less than those found for the excited states of the other Czv coinage metal trimers, CuzAg (251 and 175 cm-‘) andCuzAu (253 and 149 cm-i) [20]. In part this is simply a mass effect, since the heavier masses involved in Ag2Au will tend to cause this molecule to have lower frequency vibrations than are found in the other mol-

We gratefully acknowledge research support from the National Science Foundation under Grant No. CHE-92 15 193. Acknowledgement is also made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.

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