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

Abstract

Using femtosecond pump-probe ion imaging spectroscopy, we establish the key role of I(+) + I(-) ion-pair (IP) states in the predissociation dynamics of molecular iodine I2 excited to Rydberg states. Two-photon excitation of Rydberg states lying above the lowest IP state dissociation threshold (1st tier) is found to be followed by direct parallel transitions into IP states of the 1st tier asymptotically correlating to a pair of I ions in their lowest states I(+)((3)P2) + I(-)((1)S0), of the 2nd tier correlating to I(+)((3)P0) + I(-)((1)S0), and of the 3rd tier correlating to I(+)((1)D2) + I(-)((1)S0). Predissociation via the 1st tier proceeds presumably with a delay of 1.6-1.7 ps which is close to the vibrational period in the 3rd tier state (3rd tier-mediated process). The 2nd tier IP state is concluded to be the main precursor for predissociation via lower lying Rydberg states proceeding with a characteristic time of 7-8 ps and giving rise to Rydberg atoms I(5s(2)5p(4)6s(1)). The channel generating I((2)P3/2) + I((2)P1/2) atoms with total kinetic energy corresponding to one-photon excitation is found to proceed via a pump - dump mechanism with dramatic change of angular anisotropy of this channel as compared with earlier nanosecond experiments.

Photoionization and Photofragmentation Dynamics of I2 in Intense Laser Fields: A Velocity-Map Imaging Study

“…It has been well established that many such Rydberg states act as “doorways” to ion-pair states, ,,,,, which, assuming the dissociation of the ion-pair state is exceeded, can produce I + + I – photofragment pairs. Baklanov and co-workers have recently conducted nanosecond and femtosecond velocity-map imaging studies on the photofragmentation dynamics of 6d Rydberg states of I 2 . These experiments identified a number of dissociation channels mediated by ion-pair states, either directly to ion pair products or to electronically excited atoms through lower-lying Rydberg states.…”

“…Recent work studying the dissociation dynamics of I 2 6d Rydberg states populated by two-photon absorption in the ultraviolet have identified several such predissociation pathways with picosecond lifetimes. 36 , 38 This study reported I – /I + ion-pair dissociation products with ∼0.25 eV KER and an approximately isotropic angular distribution, in qualitative agreement with the higher KER feature observed in Figure 8 , parts c and d. The significant broadness of this feature in the current data, in addition to the additional channels at much lower KER suggest the existence of multiple ion-pair dissociations, consistent with the observation of several Rydberg states of higher energy than the 6d the photoelectron spectra ( Figure 7 c).…”

“…Assignments of several of the Rydberg states resonantly populated in the laser field are offered. Ion-pair dissociations − from these neutral Rydberg states which may remain populated following the light–matter interaction are also studied through the detection of negative I – ions, which also show different photofragmentation pathways. To our knowledge, this is the first report of anion production from molecules in intense laser fields by this mechanism, but it may be expected to be a more general phenomena in (typically halogen containing) molecules with excited states of ion-pair character. , Collectively, the results presented provide several new insights into the behavior of small molecules in intense laser fields, and furthermore establish I 2 as a fertile system for future study into molecular strong-field ionization and photofragmentation dynamics.…”

Predissociation Dynamics of Br2 in the [2Π1/2]c5d; 0g+ and [2Π3/2]c6d; 0g+ Rydberg States by Velocity Map Imaging Study

“…In this study, we have not observed a direct dissociation process in the valence states following one-photon absorption, but dissociative fragments in the one-photon energy region have been observed upon Rydberg state excitation of the iodine molecule. A similar excitation/predissociation scheme may apply for the Rydberg state of Br 2 . , …”

Photoemission resulting from collisions of I2 with 5–100 eV electrons

“…Further, it is evident from the excitation energies of I emitting states illustrated in figure 6, and shown in table 1, that the excited I species derive from I 2 Rydberg states, where the '6p' levels correlate generally to states below the I 2 ionization limit and the other I emitters to Rydberg states that converge on excited I 2 + levels. An excitation scheme that conforms to these limitations is sketched in figure 8, and was suggested in recent pump-probe experiments examining predissociation of I 2 Rydberg states [55]. In that work photo-excitation to a single, higher I 2 Rydberg level led to dissociation on several lower Rydberg asymptotes via crossings of both former and latter Rydberg curves by the same high-lying ion-pair potential.…”