J. Phys. Chem. 1988, 92, 4658-4662
4658
which are produced by the dissociation process, (5a). This is indicated by the 2.6 power dependence of the observed Hg(8d3Dl-+6p3Po) fluorescence on the laser intensity (Figure 5) and by the drastic decrease in the above fluorescence with the addition of a buffer gas in the pressure range 7-1000 mbar (Figure 6). It was also proven experimentally that excited Hg(8d3D) atoms could not be produced by a two-photon (KrF laser) excitation of ground-state Hg(61So) atoms. At low nitrogen gas pressures up to 7 mbar, collisional energy transfer assists the formation of metastable Hg(6p3P) atoms, through process 5a and with subsequent excitation increases the Hg(8d3Dl) atoms population and the resulting Hg(8d3D1-6p3Po) fluorescence, as shown in Figure 6. At higher pressures, collisional quenching of excited Hg(6p3P) atoms starts to compete with the up-pumping to the (8d3D1) state and thus results in a drastic decrease in both Hg(8d3Dl+6p3Po) and Hg(6p3Po-61So) transitions (Figure 6).
6 'P, 6
"F:
6'P,
Y Ha + Br('P1
6150
HgBr
Figure 8. Schematicdiagram of the electronic states showing the various channels of HgBrl photofragmentation.
which decay via dissociation to electronically excited HgBr molecules and Br atoms: HgBr2
- 2hv
HgBr2*
HgBr*
+ Br
(4) The fate of resulting HgBr* photofragments could be either dissociation to Hg(6p3P) and Br atoms or thermalization to HgBr(D211) molecules via electronic crossing, as presented in Figure 8, and given by the processes HgBr* Hg(63P) Br (5a)
--
+
HgBr(D) (5b) The addition of nitrogen buffer gas up to 7 mbar gives an increase in the HgBr(D-X) fluorescence (Figure 3) since collisional deactivation favors the above processes. A further increase in the nitrogen gas pressure promotes radiationless electronic-transfer processes, which deactivate HgBr molecules from the D state to the lower lying B state (Figure 8). This deactivation is more pronounced for the low vibrational levels (u' = 0-7), while the high v' levels undergo earlier vibrational deactivation. Furthermore, the formation of highly excited Hg(8d3D) atoms takes place with the absorption of an additional KrF photon by metastable Hg(6p3P) atoms: Hg(6p3P)
Hg(8d3D)
(6)
Conclusions The KrF laser photolysis of HgBrz molecules produced two strong fluorescence emissions in the visible and the ultraviolet, which are recognized as the HgBr(B-X) and HgBr(D-X) transitions. The vibrational analysis of the HgBr(D-X) transition was done and showed a high degree of vibrational excitation for the primary HgBr photofragments. The KrF laser-induced excitation produces highly excited HgBr2 molecules with a direct two-photon absorption through a real intermediate l'n, electronic state. Thus, the photofragmentation process of HgBr2 molecules occurs either from the highly excited Rydberg state or from the initially excited intermediate state and respectively leads to either excited Hg(6p3P) atoms and HgBr(D) molecules or ground-state Hg(6'So) atoms. Furthermore, collisional electronic transfer processes deexcite the initially formed HgBr(D) molecules to the lower electronic B state, from which they emit the visible (B-X) fluorescence. Nitrogen is an efficient collisional partner for the above conversion, and the maximum HgBr(D-X) emission is achieved at about 10 mbar. The Hg(6p3P-+6'So) transition is produced by the initial photofragmentation process, which yields Hg(6p3P) atoms, and the Hg(8d3Dl+6p3PI) transition is induced by the subsequent KrF excitation of Hg(6p3P) atoms. Acknowledgment. We are grateful to Y . Lazarou for his assistance in the calculation of transition wavelengths and A. Kassiotakis for his skillful technical assistance. Registry No. HgBrz,7789-47-1;HgBr, 10031-18-2; Hg, 7439-97-6; Br, 10097-32-2.
Boron Atom Reactions with Silicon and Germanium Tetrahalides C. T. Stanton, S. M. McKenzie,t D. J. Sardella, R. G. Levy, and P. Davidovits* Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167 (Received: September 28, 1987; In Final Form: February 12, 1988) Rate constants have been measured for the gas-phase reactions of boron atoms with SiF4, SiC1F3,SiCl,, SiBr,, GeF,, GeCI,, and GeBr,. The measurements were performed in a discharge flow tube apparatus at ambient temperature. The bimolecular rate constants obtained in units of cm3 molecule-' s-' are as follows: SiF,,
