Resonance enhanced multiphoton ionization photoelectron spectroscopy on nanosecond and picosecond time scales of Rydberg states of methyl iodide

Abstract

Resonance enhanced multiphoton ionization photoelectron spectroscopy on nanoand picosecond timescales of Rydberg states of methyl iodide Buma, W.J.; Dobber, M.R.; de Lange, C.A. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Rydberg states of methyl iodide have been investigated using resonance enhanced multiphoton ionization in combination with photoelectron spectroscopy with nanosecond and picosecond laser pulses. The study of the ns (6~2~ 10) Rydberg states in two-, three-, and four-photon excitations has resulted in an unambiguous identification of state [l] in the 7s and 8s Rydberg states. As a consequence, it is concluded that the transition to 6s [l] in two-and three-photon excitations is anomalously weak. The application of photoelectron spectroscopy to identify the electronic and vibrational nature of a resonance has led to a major reinterpretation of the excitation spectrum of the 6p Rydberg state in two-photon excitation. In many of the recorded photoelectron spectra anomalous electrons are observed, which derive from a one-photon ionization process. This process is suggested to find its origin in the mixing of 6p and 7s character into higher-lying Rydberg states. The major difference between resonance enhanced multiphoton ionization photoelectron spectroscopy with nanosecond and picosecond lasers is found in a less effective dissociation of the molecule in the picosecond experiments. I. INTRODUCTIONThe excited states of methyl iodide have since long attracted considerable interest, both from an experimental and a theoretical point of view.lT31 Investigations of the dissociation occurring in the lowest excited dissociative states, the A band,tm2' and the predissociation of higherlying excited states to the A band23-30 have served as benchmark studies of the dissociation of an isolated molecule. Spectroscopic studies of the Rydberg states of methyl iodide, on the other hand, have allowed for a detailed characterization of properties such as the influence of spinorbit coupling and vibronic interactions,32-35 and the study of analogies of the quantum defects in molecules and rare gas atoms.3"39Methyl iodide is a molecule of C3, symmetry with a ground state electronic configuration . + *aTe4( 'A,). In the following we will, for reasons of convenience, ...

Mass resolved MPI spectra of methyl iodide in the 430–490 nm region

“…Additionally, twophoton energies in the long-wavelength region agree with the high frequency region of A band. Much evidence ( [8], and the references therein) indicate the molecule excited to the A-band will dissociate into neutral fragment CH 3 and I, but it is not consistent with the features observed in the spectra shown in Fig. 2.…”

“…3. It can be known, in the 430-490 nm region, that three-photon energy agrees with 6p (approximately 58 000-65 000 cm − 1 ) and 7s (lies around 70 000 cm − 1 ) Rydberg states [11,12,[21][22][23][24] of the CH 3 I molecule. Four-photon energy is needed to form the CH 3 I parent ion.…”

“…Methyl iodide, as the simplest molecule of alkyl iodides and a prototypical system for experimental investigation and theoretical model of photodissociation processes in polyatomic molecules [3,4], lots of multiphoton ionization and dissociation of the molecule have been studied [5][6][7][8][9][10][11][12][13][14][15][16][17]. There still exist lots of different views about the multiphoton ionization mechanism and the assignments of some vibrational structures of Rydberg state.…”

A femtosecond velocity map imaging study on B-band predissociation in CH3I. II. The $2_0^1$21 and $3_0^1$31 vibronic levels

“…Additionally, a weak peak is seen at 10.14 eV, which could be assigned to the electronically excited ion configuration CH 3 I + (E 1/2 ). Even though core conservation is primarily expected, 13 we believe that this is the most likely assignment, since no vibrations correspond to this value of the binding energy. It is interesting to note that, for pump-only excitation at 201.2 nm, only the peak corresponding to the CH 3 I + (E 3/2 ) ground state is observed ((1 + 1) process), as had been reported in Ref.…”

“…It consists of transitions from a non-bonding 5pπ orbital of the I atom to 6s bound molecular Rydberg orbitals. Upon such excitation the molecule predissociates due to crossings with repulsive states, and therefore the spectrum shows clear vibrational structure [10][11][12][13][14] with peaks that are lifetime broadened. This system has historically received much less attention than the A-band, but a number of works have appeared in the literature recently [15][16][17] mainly exploring the details of the fast prea) Electronic mail: banares@quim.ucm.es.…”