Fourier Transform Emission Spectroscopy of the Visible Transitions of AuCl

Journal of Molecular Spectroscopy 194, 124 –127 (1999) Article ID jmsp.1998.7775, available online at http://www.idealibrary.com on

Fourier Transform Emission Spectroscopy of the Visible Transitions of AuCl L. C. O’Brien,*,1 A. L. Elliott,* and M. Dulick† *Department of Chemistry, Southern Illinois University at Edwardsville, Edwardsville, Illinois 62026 –1652; and †National Solar Observatory, 950 North Cherry Avenue, Tucson, Arizona 85726 – 6732 Received August 17, 1998; in revised form October 23, 1998

The visible electronic transitions of AuCl were observed at high resolution for the first time. The spectrum was recorded with the Fourier transform spectrometer associated with the McMath–Pierce solar telescope at Kitt Peak. The excited AuCl molecules were produced in a microwave discharge, with 4 Torr of helium seeded with 3% chlorine flowing over AuCl3 powder. Constants for the X1S1, AV 5 1, and BV 5 01 states of Au35Cl are presented. The AV 5 1 and the BV 5 01 states may be two spin– orbit components of a 3P electronic state, and molecular parameters for this excited 3P state also are given. © 1999 Academic Press INTRODUCTION

In 1928 Ferguson (1) first reported the electronic transitions of AuCl that occur in the visible region of the spectrum. Forty-three vibronic bands of two electronic transitions, A–X and B–X, were observed in the green region of the spectrum, and these were attributed to the Au35Cl and Au37Cl isotopomers. Transition energies and vibrational frequencies for the X, A, and B electronic states were reported (1). Several ab initio calculations have been published on the ground state of AuCl (2–10). The AuCl molecule is reported to have an ionic ground state, Au1Cl2, with the electron configuration Au1(5d10)Cl2(3p6) giving rise to a ground state with X1S1 symmetry (2–10). AuCl is of particular interest to theoreticians because of the large relativistic effects, which should be observable in spectra of this molecule. Comparisons of nonrelativistic calculations and relativistic calculations show changes in the ground state potential energy curve; the relativistic effects result in a shorter bond length, a higher vibrational frequency, and smaller bond dissociation energy (2–10). In the present study we report the first high-resolution observation of the AV 5 1–X1S1 and the BV 5 01–X1S1 transitions for Au35Cl. The ground state spectroscopic constants obtained in this analysis are compared with the molecular constants predicted by theoretical calculations. EXPERIMENTAL DETAILS

The excited AuCl molecules were produced in a microwave discharge, with 4.0 Torr helium seeded with 3% chlorine flowing over AuCl3 powder. The microwave power supply was operated with 50 W absorbed power. The AuCl emission was focused onto the entrance aperture of the Fourier transform 1

To whom correspondence should be addressed.

spectrometer, which is located in the McMath–Pierce solar observatory at Kitt Peak, AZ. Eleven scans at a resolution of 0.05 cm21 were co-added in 1 h of integration. A 550-nm band-pass filter limited the spectral region to 17 000 –20 000 cm21. Helium atomic lines were used to calibrate the spectrum (11). The absolute accuracy is estimated to be better than 60.005 cm21 for the spectrum. RESULTS

Eight vibronic bands were observed in the 18 000 –20 000 cm21 region of the spectrum. The vibronic bands were readily assigned using the previous work by Ferguson (1). Four vibronic bands [(0, 0), (1, 0), (2, 0), and (0, 1) bands] of the A–X transition were observed, and four vibronic bands [(0, 0), (1, 0), (2, 0), and (0, 1) bands] of the B–X transition were observed. There is little doubt that the ground electronic state has symmetry X1S1 (2–10). Assignment of the excited state symmetry for the A and B states was straightforward. One strong branch and two weak branches (Q, P, and R branches, respectively) were observed in three of the A–X vibronic bands, consistent with a transition of V 5 1 2 X1S1 symmetry. The A–X (2, 0) band is very weak. Only one branch was identified in this band and was not included in the fits. Two strong branches (P and R branches) were observed in each of the B–X vibronic bands, consistent with a transition of V 5 01 2 X1S1 symmetry. A portion of the BV 5 01 2 X1S1 (0, 0) band is shown in Fig. 1. The line positions in the spectrum were determined using the data reduction program called Gremlin2 developed by James Brault at the National Solar Observatory. For peaks with a signal-to-noise ratio greater than ;6, the peak positions were found by fitting a Voigt lineshape function to each observed

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VISIBLE TRANSITIONS OF AuCl

FIG. 1. A portion of the B V501 2X 2S1 spectrum of Au35Cl. The Au37Cl bandhead is marked with an asterisk.

spectral feature. For weaker peaks, the peak centers were estimated by eye. A total of 1008 lines were used in a combined fit to obtain a total of 25 molecular constants using a nonlinear leastsquares fitting program. All lines were weighted equally except those badly obscured by atomic lines or severely blended with other AuCl lines. The average uncertainty as determined from the standard deviation of the fit was 0.006 cm21, which was consistent with the estimated measurement accuracy. The standard effective Hamiltonian expressions were used for the 1S1 state, the V 5 01 state, and the V 5 1 state (12–13). The molecular constants for the X1S1, the AV 5 1 state, and the BV 5 01 state of Au35Cl are presented in Tables 1, 2, and 3, respectively. Line positions, assignments, and fit residuals can be obtained from the Journal of Molecular Spectroscopy Depository. Chlorine has two naturally occurring isotopes, 35Cl (75% abundant) and 37Cl (25% abundant). Due to the relatively low

signal-to-noise ratio (S/N ' 15 for the strongest branches of Au35Cl in the 0 – 0 band) and the congested nature of the spectrum, we were only able to analyze the Au35Cl bands. Short portions of the Au37Cl branches could be identified, but so many lines were obscured that a secure assignment was not possible. However, the Au37Cl bandheads were readily observed in our spectrum, as illustrated in Fig. 1. DISCUSSION

The observed vibronic bands of AuCl are in very good agreement with the bandheads reported by Ferguson (1). Only two ground state vibrational levels were observed at high resolution in our spectrum, n 5 0 and n 5 1. Using the ground state ve xe value from Ferguson (1), the ground state vibrational frequency can be calculated, ve 5 383.30 cm21. This agrees TABLE 2 Spectroscopic Constants (in cm21) for the AV 5 1 State of Au35Cl

TABLE 1 Spectroscopic Constants (in cm21) for the X1E1 State of Au35Cl

Note. One standard deviation error on the last digits is quoted in parentheses.

Note. One standard deviation error on the last digits in parentheses.

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O’BRIEN, ELLIOTT, AND DULICK

TABLE 3 Spectroscopic Constants (in cm21) for the B V 5 01 State of Au35Cl

Note. One standard deviation error on the last digits is quoted in parentheses.

well with the value from Ferguson (1), ve 5 382.8 cm21. Comparisons of the observed ground state vibrational frequency to the predicted vibrational frequency from two recent, relativistic ab initio calculations are presented in Table 4. The bond length of the ground state, re, also can be calculated. From the relationship Bv 5 Be 2 ae(n 1 21), Be 5 0.117388(11) cm21 and ae 5 0.5431(92) 3 1023 cm21; this yields re 5 2.19903(21) Å. The ground state bond length is compared with several recent, state-of-the-art relativistic ab initio calculations in Table 4. Both the MP2 calculation (3) and the LDF method calculation (2) give reasonably accurate predictions for the ground state bond length. Compared to nonrelativistic calculations, relativistic corrections reduce the bond length by 0.2– 0.4 Å (cf 2–10). The ground electronic state of AuCl is well understood (2–10). The parentage of the excited states, however, is not entirely clear. The AV 5 1 and the BV 5 01 states are about 125 cm21 apart and have very similar spectroscopic constants

FIG. 2. Molecular orbital diagram for AuCl.

(see Tables 2 and 3), so it is quite likely that they could be the spin– orbit components of a single electronic state. A simplistic molecular orbital diagram for the ground state electron configuration of AuCl is presented in Fig. 2. Promotion of a chlorine 3pp electron to the gold 6s orbital (see Fig. 2) would give rise to two excited electronic states with symmetries 1P and 3P. In Hund’s case c, the 3P state produces V 5 01, V 5 02, V 5 1, and V 5 2 states. One other possible electronic transition would be the promotion of a chlorine 3ps electron to the gold 6s orbital, which would give rise to two excited electronic states, with symmetries 1S1 and 3S1. Since the 3S1 state produces Hund’s case c V 5 02, and V 5 1 states, the 3S1 state can be removed from consideration because 02– 01 is not allowed (12). Thus the excited electronic states we have observed could be the excited 1P and 1S1

TABLE 4 Comparisons of Ground State Bond Length (in Å) in Vibrational Frequency (in cm21) with ab initio Calculations

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TABLE 5 Spectroscopic Constants (in cm21) for the Excited 3P State of Au35Cl

Note. One standard deviation error on the last digits is quoted in parentheses. * Held fixed, see text.

states, or they could be the V 5 1 and V 5 01 components of the 3P state. We favor assigning our observed excited states to a single 3P state. Based on the molecular orbital diagram given above and Hund’s rules (12–13), the 3P state is expected to be the lowest excited state. Indeed, Ferguson searched the entire visible and ultraviolet regions, 700 –200 nm, and found only the electronic transitions described above which occur in the green (1). The observed transitions also have been fit as (0, 0), (0, 1), and (1, 0) vibronic bands of a 3P–X1S1 transition. [The BV 5 01 2 X1S1 (2, 0) band was not included in the fit.] The fitting program incorporated the standard effective Hamiltonian for a 3 P state, with explicit matrix elements from Brown and Merer (14) and Brazier et al. (15). For the final fit, AD was constrained to the value calculated by the following formula given by Veseth (16):

would imply an effective spin– orbit constant of Aso 5 2125.34 cm21. However, since both gold and chlorine are relatively heavy atoms, it is possible that the spin–spin parameter, l, is very large and negative. This would shift the V 5 0 and V 5 1 components closer together by 22l (15). For example, if l 5 2225 cm21, the new effective spin– orbit constant for the 3P state would be Aso 5 2575.31 cm21. Thus from the data available, we can determine only the difference between the two parameters, Aso 2 2l 5 2125.34 cm21. The 3 P lambda-doubling parameter o could not be determined from the available data, and also is included in the effective spin– orbit constant. The excited state bond length can be calculated as described above. The excited state rotational constant was calculated, Be9 5 0.10820 cm21, and the excited state bond length was calculated, re0 5 2.291 Å. The observed electronic transition, 3P [Au(5d106s1)Cl(3p5)]–X1S1[Au1(5d10)Cl2(3p6)], is a good example of a charge transfer transition, which also explains the large increase in the bond length in the excited state (re9 5 2.291 Å and re0 5 2.199 Å). ACKNOWLEDGMENTS Partial support for this work was provided by the National Science Foundation9s Professional Opportunities for Women in Research and Educations Program, through Grant NSF-CHE-9753254. The authors thank C. R. Brazier for helpful discussions on the source code for the 3P Hamiltonian matrix.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

AD 5 2 AJ 5

2~ A n11 2 A n! D n50 5 21.39 3 10 24 cm21 B n 2 B n11 1 6B 2e / v e

The molecular constants for the excited 3P state of Au35Cl are presented in Table 5. For the excited state electron configuration, 3P Au(5d106s1) Cl(3p5), the molecular spin– orbit constant should be equal to the chlorine atomic spin– orbit constant, z 5 2587.3 cm21 (13). However, the 3P0 and 3P1 spin– orbit components are observed to be separated by 2125.34 cm21 (the 3P0 component lies higher in energy than the 3P1 component), which

9. 10. 11. 12. 13. 14. 15. 16.

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