Spectroscopy and photophysics of 1Πg and 3Πg Rydberg, ion-pair states of Cl2, revealed by multiphoton ionization

Abstract

Mass-resolved, two-photon resonant, three-photon ionization of jet-cooled Cl2 has been used to study the spectroscopy and photophysics of the lowest energy, double-well 1Πg and 3Πg states of mixed Rydberg (4sσ) and ion-pair character. Inner well levels from v′=0 (64 027 cm−1) to v′=15 (73 343 cm−1) have been observed for 1Πg of the mass 70 molecular chlorine isotope. Inner well levels for 3Πg have been observed from v′=0 at 63 472 cm−1 to the top of the barrier, and well beyond that to levels spanning both wells. Observation of these long progressions is attributed to excitation via a repulsive continuum at a well-phonon level. The photophysics of this latter process as well as the mechanism for copious atomic ion formation are discussed. Variation of linewidth with vibrational quantum number due to the predissociation of inner well levels is observed and attributed to repulsive valence states. The data are in good agreement with the theoretical calculations of Peyerimhoff and Buenker, and Tuckett and Peyerimhoff.

Mass-resolved multiphoton ionization spectroscopy of jet-cooled Cl2. I. Bound-free-bound spectroscopy

“…11 In particular, it was proposed that the crossing of the inner wall of the 3 ⌸ g ion-pair states with the outer wall of the triplet 4s Rydberg states would result in double-minimum potentials. Li et al 12 suggested that results from their mass-resolved one-color ͑2ϩ1͒ resonanceenhanced multiphoton ionization ͑REMPI͒ studies supported this theory. Their spectra contained a long vibrational progression of red-degraded bands between 70 000 and 74 000 cm Ϫ1 , provisionally assigned as having 1 g symmetry, with spacings indicative of an ion-pair state based on Cl ϩ ͑ 3 P J ͒ ͑i.e., one from the first cluster͒.…”

“…3 was previously assigned 12 as arising from the ␤͑1 g ͒ state. This assignment can now be confirmed from the current OODR results which overlap both the earlier low vЈ data and the REMPI data.…”

“…Previous REMPI spectra 12,13 have shown a second very unusual feature in the energy range 64 000-74 000 cm Ϫ1 . Transitions are seen from vЉϭ0 of the ground state to vЈϭ0-15 of the ͓ 2 ⌸ 1/2 ͔ c 4s; 1 g Rydberg state.…”

“…The extended progression seen following two-photon excitation contrasts with the results of electron energy loss studies 14 where only vЈϭ0-6 bands were observed, with an intensity maximum at vЈϭ2. It was concluded 12 that the unusual two-photon behavior was again due to the C state, present at the onephoton level, causing some bond stretching in the overall two-photon transition. The criteria necessary for the observation of transitions via a repulsive intermediate state 2 which result in bond stretching are applied here in the analysis of the ͓ 2 ⌸ 1/2 ͔ c 4s; 1 g Rydberg state.…”

“…Our current OODR results confirm their assignment, extend their data and give the absolute vibrational numbering. It was further proposed 12 that only those red-degraded bands above the barrier of the double-minimum potential were observed. In the present paper we reinvestigate the region of this proposed avoided crossing in the light of the OODR and REMPI data which extend up to 78 000 cm Ϫ1 .…”