Photoionization spectroscopy of NaAg

21 February 1997

CHEMICAL PHYSICS LETTERS

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ELSEVIER

Chemical Physics Letters 266 (1997) 189-194

Photoionization spectroscopy of NaAg A. Stangassinger, A.M. Knight, M.A. Duncan * Department of Chemistry, Universityof Georgia, Athens, GA 30602, USA Received 10 October 1996; in final form 11 December 1996

Abstract

Two electronic systems of NaAg are observed with resonant photoionization spectroscopy, a weak one near 300 nm and a much stronger one at 330 nm. The 300 nm system correlates to the dipole-forbidden 3s-4s (2S ~ 2S) transition of sodium and the stronger transitions converge to the 3s-3p (2p ,__ 2S) transition of sodium. Vibrational constants of three upper states and the ground state are reported. 39 bands are observed for the weak G ~- X transition originating from v" = 0-3 ground state vibrational levels. The ground state binding energy is greater than or equal to 1.59 eV and the vibrational frequency (to~) is 210 cm -I. 1. I n t r o d u c t i o n Metal diatomics provide diverse examples of chemical bonding. Electronic spectroscopy of metal dimers produced and studied in the gas phase has provided detailed information on bond energies, bond distances and electronic configurations [1-3]. Theory has also investigated metal-metal bonding, with the most success for diatomics with limited numbers of valence electrons. Thus, calculations on alkali metal or coinage metal dimers are more tractable than those on transition metals. Electronic spectra for alkali or coinage metal dimers are likewise easier to interpret because there are fewer overlapping electronic states. Therefore, recent experiments and theory have focused on heteronuclear dimers containing these metals for comparison to the previously known homonuclear dimers [2,4-12]. In the present study, we present the first spectroscopy on the new alkali-

* Corresponding author.

coinage mixed-metal dimer, NaAg. This system becomes an addition to the series of these dimers (LiAg [11,12], KAg [5], LiCu [11]) which we have studied recently. Recent work in other labs has investigated mass spectra of N a / A g clusters [13], but there is no previous spectroscopy on these systems. The ground electron configuration of the Ag atom is d~°s ~. The d9s 2 configuration is much higher in energy, which essentially eliminates d-orbital participation in ground state bonding [14]. The ground electronic configuration of the Na atom is s ~, which also severely limits its available orbital participation. The ground state of the mixed dimer then should be JE+ resulting from an s - s cr bond. However, while the homonuclear diatomics of these atoms necessarily exhibit covalent bonding, the mixed dimer may have ionic character resulting from the large electronegativity difference of the constituent atoms. The degree to which this is important can be estimated by a comparison of the mixed dimer bond energy, bond distance and vibrational frequency to the corresponding values for the homonuclear dimers. In the systems studied previously, the properties of LiAg,

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LiCu and KAg are all consistent with significant ionic bonding. Similar effects are expected and observed here for NaAg.

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2. Experimental Alkali-noble metal alloys are not readily available, and so a critical aspect of this work is sample preparation. In previous work, a dual oven source was described to produce mixed N a / A g clusters [13], but such a source is inconvenient for our pulsed molecular beam apparatus. We therefore use laser vaporization of rotating rod samples which contain both elements. Samples containing both sodium and silver are prepared by 'salting' a silver rod with NaC1. In this procedure, salt is rubbed into the surface of a silver rod with a plastic scrubbing pad. The method of thin film alkali metal vapor deposition, which we e m p l o y e d previously for lithium/silver [ll], is not successful for sodium. Sodium does not bind to a silver surface and is easily oxidized, therefore tending to flake off the sample. An excimer laser (Lumonics TE-860-4 excimer) at 308 nm (XeCI) is used to vaporize the salted sample, producing both atoms in the resulting plasma. This sample is particularly sensitive to the intensity of the vaporization laser ( < 30 m J/pulse) and the speed of rod rotation. The plasma is cooled in a pulsed supersonic expansion with He carrier gas at 60 psig back pressure. The molecular beam then enters the extraction region of the time-of-flight mass spectrometer, where photoionization occurs. The molecular beam machine and mass spectrometer have been described previously [15]. For the low resolution (0.5 cm - I ) study reported here, a frequency doubled Nd:YAG pumped dye laser system (Spectra-Physics GCR-3 + PDL-2) is used. Spectra are recorded in two mass channels (23Nal°9Ag and 23Nal°7Ag) to detect isotopic shifts. Spectra are calibrated with the atomic transitions of Na and Ag.

3. Results and discussion

Two electronic band systems are observed for NaAg, as shown in Figs. 1 and 2. In both systems, spectra are observed in the parent ion mass channel

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3185B 3285e 3225B 3~.454B 3265e 3285e 33850 33258 3345e energ

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(cm -1)

Fig. I. The G '--- X band system observed for 23Nal°VAg near 300 r i m .

for both silver isotopes, confirming the mass assignment. The spectrum in the 300 nm region is very weak in intensity, but somewhat simpler in structure, while the spectrum near 330 nm is about an order of magnitude more intense and much more complex. The position and signal strengths in these two regions suggest that the 300 nm system correlates to the dipole forbidden Na 4s ( 2 S ) + Ag (2S) excited state at 25740 cm-~ and that the 330 nm system correlates to the Na 3p ( 2 p ) + Ag (Zs) excited state at 16956/16973 cm-~. No other assignments for the corresponding atomic transitions leads to a consistent set of dissociation energies, as discussed below. We first investigate the 300 nm system, which has many clearly resolved vibronic bands. Our scans in this region cover the energy range from 31850 erato 33450 cm -~. The spectrum appears noisy due to the extremely low signal levels. Four different progressions are assigned, as shown in Fig. 1, which belong to the same electronic state starting from different ground state vibrational levels. The band positions observed in the 23Nal°TAg mass channel are given in Table 1. The isotope shift observed for these bands is quite small, even for the higher vibrational levels. An unequivocal numbering of the upper vibrational levels is therefore not possible. However, the best fit is obtained by assigning the lowest

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A. Stangassinger et al. / Chemical Physics Letters 266 (1997) 189-194

t~Ag

_

mNa

8

8

d

i

283B8

28558

28888

2985e

2938B

2955B

29688

literature value (1.40 eV), which was obtained via a thermochemical study [17]. However, there is considerable uncertainty in this value caused by the Morse potential assumption, the significant extrapolation and the poorly defined vibrational numbering. Three hot progressions were also found in the G ~ X band system, whose assignments are given in Table 1. Using these progressions, the ground state vibrational frequency is found to be oJ~ = 210 c m - t. The lower energy region was investigated from 28300 to 32000 c m - 1, revealing the spectrum shown in Fig. 2. This signal is also observed in the parent ion channel with the corresponding silver isotopic structure. The signal in this region is at least an order

energ~il (cm-t)

Fig. 2. The complex overlapping band systems observed for NaAg near 330 nm. The spectrum here is about 10 times more intense than that shown in Fig. 1.

observed band position at 31998.8 cm -1 to the v' = 0 ~ v" = 1 transition. This assignment results in an excited state vibrational frequency of ~o'e = 124.7 c m - i and an anharmonicity of %x'e = 1.16 cm - l . Using these constants, we use a Morse potential extrapolation [16] to derive an excited state dissociation energy of D~ = 3289 cm - l . The spectroscopic constants for N a A g are given in Table 2, This dissociation energy and the position o f the spectrum indicate that the excited state correlates to an asymptote lying no higher than 35500 cm - t . The atomic states of both silver and sodium in this region are well known. The extremely weak intensity indicates that the transition correlates to the forbidden 2S ~ 2S (4s ~ 3s) sodium atom transition. W e therefore indicate the molecular transition as G ~ X. In counting states in this way, we include both singlet and triplet states correlating to the various atomic asymptotes, as would be appropriate for a H u n d ' s case c) system. An estimate of the ground state dissociation energy can be calculated with this data and the formula %o + D~ - Tat ~- D~, where the v0o value is extrapolated and T~, indicates the atomic asymptote. The value obtained for the ground state binding using the G ~ X data is D~ = 1.22 eV. This is lower than the previously reported

Table I Band positions of the G ,,--X transition of 23 Na ~°TAg U', /f'

cm-

0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0 12,0 0, 1, 2, 3, 4, 5, 6, 7, 1,2 2,2 3,2 4,2 5,2 6,2 7,2 8,2 3,3 4,3 5,3

(32204.1) 32327.4 32444.3 32562.1 32677.9 32791.8 32903.2 33012.0 33119.8 33224.6 33324.0 33421.3 33512.7 31998.8 32117.7 32235.5 32353.4 32467.2 32581.1 32692.9 32802.8 31909.0 32028.8 32145.7 32259.5 32373.3 32485.2 32594.1 32700.9 31936.9 32051.8 32164.6

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A. Stangassinger et al.// Chernical Physics Letters 266 (1997) 189-194

Table 2 Vibrational constants for the 23Na ~°7Ag isotopomer State

VGO( c m - I )

toe ( c m - I )

X A B G

0 28308 28361 32204

210 85.8 82.3 124.7

toex e ( c m - I )

D o(cm

1.38 1.47 1.16

12827 ! 334 1152 3289

i)

of magnitude more intense than that of the G *-- X system, but the spectrum is quite congested. Only in the beginning of the spectrum at low energy are we able to find two clear progressions with more than three upper vibrational levels. Although the intervals between these progressions are fairly constant, we are not able to definitively assign them to a common system. We therefore assign these to two separate A *-- X and B ~ X band systems. As in the G ~ X system above, isotopic shift analysis of these bands is inconclusive, and we therefore make our assignment without complete confidence. The tentative spectroscopic constants for the A and B excited states are given in Table 2. The observation of one-color two-photon ionization at the A *-- X origin at 28308 cm-J indicates that the ionization potential of NaAg is ~< 56616 c m - ~ (7.02 eV). The position and intensity of the absorption in this region of the spectrum are consistent with molecular systems correlating to the strongly allowed Na 2p *--2S atomic transition. At this Na ( 2 p ) + Ag (2S) excited asymptote, four molecular states are expected (IE, 3E, 1II, 317). In the 'small molecule' Hund's case a) approximation, singlet triplet transitions are forbidden, and only two band systems should be observed here ( ' E ~ X ' X and 11] ( - ' - X I E ) . However, the extreme complexity in this region appears to indicate that there are more than two band systems here. This suggests that it is more appropriate to regard NaAg as a Hund's case c) system, where additional band systems corresponding to singlet-triplet transitions become allowed. This is consistent with previous work in our lab, where both AIAg [8] and LiAg [11] exhibited case c) behavior. While vibronic analysis in the 330 nm region is frustrating, the spectrum here does provide useful information about the bond energy of the NaAg molecule. As shown in Fig. 2, absorption is observed

from about 28300 to 29800 cm -~. The molecular band systems in this region must correlate to the Na 2p ~ 2S atomic transition. Any other choice for the atomic asymptote does not give a ground state bond energy remotely consistent with that derived from the G ~ X band system or with the previous thermochemical value. It is then immediately apparent that the ground state bond energy must be larger than the 1.22 eV value determined from the G ~ X system. Were this not the case, the bands at 28300 would already lie above the Na ( 2 p ) + Ag (2S) asymptote energy. The observation of absorption up to at least 29800 c m - ! indicates that the ground state is even more strongly bound. Bound levels at this energy for a state correlating to the Na (2p) asymptote implies that the ground state is bound by at least 2 9 8 0 0 16973 = 12827 cm -I (1.59 eV). The loss in absorption intensity near 29800 cm-1 could be caused by the end of the Franck-Condon intensity profile for states whose bound levels extend higher in energy. If this is the case, the ground state bond energy must be still greater than 1.59 eV. On the other hand, the intensity drop-off is rather sudden. It seems unlikely that the Franck-Condon profiles of several electronic systems expected in this region would all end simultaneously, unless this position is the convergence limit indicating the exact asymptote position. If this is the case, the convergence limit lies at 29800 cm -~, and subtraction of the atomic asymptote energy as shown above gives the exact ground state bond energy of 1.59 eV. We suspect that the latter scenario is correct, but without direct evidence to support this we report the value as a rigorous lower limit. This value is much greater than the 1.40 eV thermochemical value determined previously [17]. The observation of a convergence limit such as this makes it possible to determine the dissociation energy without any assumption about the nature of the excited state potential except that it must not have a hump or barrier at long range which could give rise to a false convergence limit. We believe that this latter possibility is unlikely. The NaAg dissociation energy is compared to the properties of Ag 2 and Na 2 in Table 3. From the data in Table 3 it can be seen that the ground state vibrational frequency of NaAg is larger than the values for the homonuclear diatomics Na 2 and Ag 2 [3]. Also, the dissociation energy is inter-

A. Stangassinger et al. / Chemical Physics Letters" 266 (1997) 189-194

Table 3 Comparison of spectroscopic constants for the homonuclear diatoms Ag z and Na 2 with the heteronuclear dimer NaAg l°TAg2 Na t°VAg Na 2 we (cm- i) r e' (,~) D~ (eV)

192.4 2.53 1.66

210 1.59

159.1 3.08 0.72

mediate, but very close to that of Ag 2- These are the same trends observed previously for LiAg, LiCu and K A g [8,11] and they suggest that there is a significant ionic contribution to the bonding in NaAg. As discussed by Pauling [18], the percent ionic character of a heteronuclear diatomic with a single bond can be estimated by: % = 1 - e x p [ - ( X a where the X values are electronegativities for the respective atoms. The values for silver and sodium are: )tAg = 1.93 and XNa = 0.93 [19], resulting in a calculated ionic character for N a A g of about 22%. This suggests that there is a considerable N a ÷ A g ionic contribution to the ground state bonding. Ionic character is reasonable for this system because of the low ionization potential of sodium (5.139 eV) and the high electron affinity of silver (1.302 eV). The resulting difference I P ( N a ) - EA(Ag) is 3.84 eV. This is the energy separation between the ion pair asymptote and the asymptote of neutral ground state atoms. As shown in Table 2, the dissociation energy of the N a A g ground state is also much greater than the values for the low lying excited states, to the extent that our approximate numbers are valid. This is also understandable, since the ground state is ~ ; , and this state can mix effectively with the ~E ion pair potential. If our correlations are correct, the excited states observed here all correlate to atomic asymptotes which have excited sodium atoms and ground state silver atoms. Therefore the excited electron density in the molecular states is more likely to be concentrated on the sodium atom. These configurations are therefore less likely to mix effectively with the ion pair potential, which has no valence electron density on sodium. In summary, the first electronic spectra observed for the new diatomic N a A g are reported, The spectrum is congested, and significant portions of it are -

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complex beyond our ability to obtain satisfying analysis. Nevertheless, the G ~ X system is analyzed, yielding the excited state and ground state vibrational frequencies. The limited number of atomic asymptotic states possible in this molecule make it possible to associate absorption systems near 300 nm and 330 nm with specific atomic asymptotes. In so doing, we are able to obtain the ground state dissociation energy, which is significantly greater than the previous literature value. The dissociation energy and vibrational frequency indicate that N a A g has significant ion pair configuration mixing in its ground electronic state, as was observed previously for the related diatomics LiAg, K A g and LiCu. Future studies may focus on additional electronic states expected at higher energy.

Acknowledgements

This research is supported by the US Department of Energy through grant No. DE-FG05-93ER14402.

References

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[13] U. Heiz, A. Vayloyan and E. Schumacher, J. Phys. Chem. 100 (1996) 15033. [14] C.E. Moore, National Standard Reference Data Series 35. Atomic energy levels (National Bureau of Standards, Washington, DC, 1971). [15] K. LaiHing, R.G. Wheeler, W.L. Wilson and M.A. Duncan, J. Chem. Phys. 87 (1987) 3401. [16] G. Herzberg, Molecular spectra and molecular structure I.

Spectra of diatomic molecules (Van Nostrand Reinhold, New York, 1950). [17] V. Piacente and K.A. Gingerich, High Temp. Sci. 4 (1972) 312. [18] L. Pauling, The nature of the chemical bond (Cornell University Press, Ithaca, New York, 1960). [19] A.L. Allred, J. lnorg. Nucl. Chem. 17 (1961) 215.