Spectroscopy and Predissociation of Acetylene in the np Gerade Rydberg States

Abstract

The np gerade Rydberg states of acetylene were studied with various spectroscopic techniques to measure one-color resonant multiphoton ionization (REMPI) spectra, two-color REMPI mass spectra, fluorescence spectra, and fluorescence excitation spectra in the two-photon energy region of 72 000-92 000 cm -1 . The rotational analysis was successfully performed for one of the upper levels, which was assigned to the 4p 1 ∆ g Rydberg state. All of the gerade Rydberg states prepared under the two-photon resonance conditions were found to be predissociative, and the two-photon preparation was accompanied by the fluorescence of the C 2 d 3 Π g f a 3 Π u Swan system. In the fluorescence excitation spectra measured by monitoring the C 2 Swan system, higher members of the np 1 Σ g + and 1 ∆ g Rydberg series (n g 4) were observed and assigned for the first time. The predissociative lifetimes of the np gerade Rydberg states are estimated from the spectral bandwidths. The mechanisms of the predissociation of acetylene in the Rydberg states and the formation of C 2 in the d 3 Π g state are discussed.

Analysis of Low-Lying Gerade Rydberg States of Acetylene Using Two-Photon Resonance Fluorescence Excitation Spectroscopy

“…This conclusion is in accordance with our previous assignments. 21 The quantum defects calculated for the Rydberg states of C 2 D 2 are almost the same as those for C 2 H 2 and roughly classified into three values; 0.5, 0.6, and 1.1 corresponding to npπ 1 Σ g + , npπ 1 Δ g , and npσ 1 Π g , respectively. Considering quantum defects, vibrational frequencies, and band intensities, we assigned the diffuse bands to transition to the npπ gerade Rydberg states of C 2 D 2 and C 2 H 2 as listed in Table 2.…”

“…20 In our previous study, we found the 4pπ 1 Δ g and 4pπ 1 Σ g + states of acetylene using one-color (2 + 1) REMPI spectroscopy. 21 We found that the lifetime of the 0°level in the 4pπ 1 Δ g state was sufficiently long to record the rotationally resolved spectrum. We also found that the photofragment C 2 emission of Swan bands (C 2 d 3 Π g −a 3 Π u ) was induced by the multiphoton absorption via the two-photon resonance gerade Rydberg states of C 2 H 2 , and we succeeded in recording a wider energy range of the C 2 H 2 excitation spectrum.…”

“…The 0 0 0 band of the C 2 H 2 4pπ 1 Σ g + −X ̃1Σ g + transition showed a completely broad feature without any rotational structure and was reproduced by a simple Lorentzian profile. 21 This apparent discrepancy between two isotopic molecules is reasonable if we consider the isotope effect on the vibrational frequencies and vibronic structures because quantum interference strongly depends on the energy gap between the interacting states.…”

“…The experimental setup used in this study has been described previously. 21 The excitation spectra of C 2 D 2 were obtained by monitoring the total Swan band emission of the C 2 fragment. Fluorescence was detected by a photomultiplier tube (Hamamatsu, R-105 and R-928), with optical filters (Toshiba, UV-29 and Y-44) to select visible light emissions in the 450−650 nm region.…”

“…All C 2 D 2 two-photon absorption bands observed in this study are compared with those previously observed for C 2 H 2 . 21 The 0 0 0 bands can easily be identified by comparing the band positions and intensities. For example, the partially structured C 2 D 2 band at 86 407 cm −1 corresponds to the C 2 H 2 86 380 cm −1 band, which also shows a partially resolved Δ−Σ type rotational structure.…”