Neutral photodissociation of superexcited states of molecular iodine

Abstract

The formation of high-n Rydberg atoms from the neutral dissociation of superexcited states of I2 formed by resonant two-photon excitation of molecular iodine using an ArF laser has been investigated. The high-n Rydberg atoms I* are formed by predissociation of the optically excited molecular Rydberg states I2 * RB 2g + converging on the I2 +B 2g + state of the ion. Measurement of the kinetic energy release of the Rydberg I* fragments allowed the identification of the asymptotic channels as I*R3PJ+I2P3/2, where the I*R3PJ are Rydberg atoms converging on the I+3PJ states of the ion with J=2, 1, and 0. In the case of the I*R3P2 fragments, the average Rydberg lifetime is observed to be 325±25 s. Based on experiments on the variation of the Rydberg atom signal with the field ionizing strength, the distribution of Rydberg levels peaks at about 25–50 cm−1 below the ionization limit

Role of ion-pair states in the predissociation dynamics of Rydberg states of molecular iodine

“…They can be coupled by radiation in a one-photon ''parallel'' process with states 0 u and 2 u , respectively. States of this symmetry exist among the repulsive valence-shell states of I 2 correlating with the formation of a pair I( 2 P 3/2 ) + I( 2 P 1/2 ) in process (6). 25 This further supports the interpretation that these states can act as intermediates in a direct twophoton excitation, as we have argued above.…”

“…Our hypothesis about the participation of intermediate states of different symmetries is supported by the results obtained for channel C6 which corresponds to photodissociation after excitation of I 2 by one photon (6). Fig.…”

“…From a classical simulation we infer that this is not due to an averaging effect caused by the Fig. 11 Images displaying the difference in angular anisotropy of recoil directions for I( 2 P 1/2 ) fragment atoms with a TKER value corresponding to process (6) rotation of the excited molecule. To explain this value of the anisotropy parameter we suppose that direct two-photon excitation via multiple pathways involving intermediate states of different symmetry dominates over a stepwise process.…”

“…A very useful approach for the identification of dissociation channels is based on measuring the photofragments distribution as a function of kinetic energy release (KER), which has been applied to the study of I 2 photodissociation via high-lying states. 6,7 O'Keeffe and coauthors investigated photofragments appearing after twophoton excitation of I 2 at 193 nm into superexcited Rydberg states converging to the excited electronic state of the molecular ion I 2 + (B 2 S g + ). 6 Measurements of KER allowed these authors to conclude that these superexcited states dissociate via neutral channels giving rise to the pair I[R( 3 P J )] + I( 2 P 3/2 ), where I[R( 3 P J )] are Rydberg atoms converging to the I + ( 3 P J ) states of the ion with J = 2, 1, 0.…”

Halogens and noble gases

“…The resulting formation of high-n Rydberg fragments originates from the neutral dissociation of these superexcited states. 9 This method is complimentary to previous studies using fluorescence or photoelectron spectroscopy which favor the detection of low-n Rydberg fragments. In another report, the vibrational dynamics of the B 3 P + u state of molecular I 2 , as well as the ion pair states accessible from there by a one-photon transition, were observed using Femtosecond degenerate fourwave-mixing spectroscopy using an initial pump laser pulse.…”