On the B <- X transitions of expansion cooled silver halides

摘要

Fluorescence excitation and mass-resolved excitation spectroscopy of jet-cooled silver halide (AgX) bare molecules are explored for the B <-- X systems of AgCl, AgBr, and AgI. These spectra are compared to those from static gas measurements. The continuous-wave expansion molecular beam of Ar-He with AgX species is generated at a temperature of 1100 degrees C. For AgI two types of progressions are shown to appear in 31000-33000 cm(-1) range. The first set of transitions is attributed to the B state and is quite similar to the other AgX molecule's B <-- X transitions. The second set has been previously observed for high-temperature AgI vapor as a partly resolved series of features on a broad absorption. The nature of this state and whether it arises from a level crossing between the B surface and an unobserved state is not clear. We analyze two different possible explanations for the newly resolved B' progression: The B and B' progressions involve two different excited electronic states; and both progressions belong to one excited electronic state with a complicated single adiabatic potential surface. Computation of vibrational harmonic and anharmonic constants suggests that the potential shape of the B/B' state is substantially different from that of a common anharmonic oscillator. The nature of the B/B' surface must he resolved through detailed and accurate ab initio calculations. Ag ions produced by 1 + 2 photoionization of AgX at longer wavelengths are not fragmentation products of AgX ions, but rather arise from the dissociation of a neutral AgX in a highly excited state. This intermediate state at similar to 62000 cm(-1) is accessed from the B state by absorption of a photon of approximately the same energy as required for the B <-- X AgX transition. (C) 1998 American Institute of Physics. [S0021-9606(98)00946-5]. [References: 34]