This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21, 10391--10401 | 10391 Cite this: Phys. Chem. Chem. Phys., 2019, 21, 10391 Two-color studies of CH 3 Br excitation dynamics with MPI and slice imaging† Arnar Haflijason, a Pavle Glodic, b Greta Koumarianou, b Peter C. Samartzis * b and A ´gu ´ st Kvaran * a Two-color pump–probe experiments were performed to explore the multiphoton dynamics of CH 3 Br at high excitation energies of 8–10 eV, involving two-photon resonant excitations to a number of np and nd Rydberg states (pump) followed by REMPI detection (probe) of the Br, Br* and CH 3 (X) photoproducts. Slice images of Br + and CH 3 + ions were recorded in pump-only, probe-only and pump and probe experiments. Kinetic-energy release spectra (KERs), as well as spatial anisotropy parameters, were extracted from the images to identify the processes and the dynamics involved. Predissociation channels, following the two-photon resonant excitations and non-resonant photodissociation forming CH 3 (X) and Br/Br*, were identified and characterized. Furthermore, the probe excitations for CH 3 (X) involved near- resonant excitations to lower energy 5s Rydberg states of CH 3 Br. In three-photon excitation processes, a striking contrast is seen between excitations via the p/d and the s Rydberg states. Involvement of high- energy interactions between Rydberg and ion-pair states is identified. I. Introduction Significant emphasis has been placed on the photodissociation dynamics of the halocarbons for decades; in particular for the methyl monohalides. 1–13 Whereas methyl iodide (CH 3 I) has attracted most attention, 14–28 substantial work has also been done on methyl bromide (CH 3 Br). 1–5,7,10–12,19,20,29–32 Most of those studies have explored the dynamics of repulsive valence states in the A-band spectral region. 3–9 Less is known about excitations to higher energies where Rydberg states dominate the spectrum. 1,2,10–13 Laser excitations by the method of resonance enhanced multi-photon ionization (REMPI) are suitable to access Rydberg states conveniently by use of visible or near-UV radiation. 1,2,10,12,13,21,22,24,25,30,33–36 Furthermore, due to selec- tion rules, more states may be accessed by multiphoton excitation than by single-photon absorption. 37–40 State inter- actions gradually increase with excitation energy as the density of states increases. This can cause spectral perturbations such as line shifts and/or line broadening. 41–46 Interactions between CH 3 Br Rydberg states and the ion-pair/valence state, CH 3 + Br  , could be involved in photoion pair formation, analogous to observa- tions for some halogen-containing diatomic molecules. 47–51 By further analogy with observations for the hydrogen halides there is reason to believe that Rydberg to ion-pair interactions are important in predissociation processes. 44,45,52–56 REMPI data of CH 3 Br for one-, two- and three-photon resonant excitations to a series of Rydberg states have been reported. 2,10,13 In (2 + m) REMPI, formation of CH 3 + ions is found to dominate whereas parent ion signals are negligible and Br + signals are very weak. Here, (2 + m) represents two- photon resonant excitation to intermediate (Rydberg) states followed by m-photon ionization to form M + for M = CH 3 , CH 2 , CH, Br or CBr. The intensities of fragment ion signals typically vary as CH 3 + 4 CH 2 + 4 CH + 4 (Br + ,CBr + ). 1,10 Slice imaging experiments reveal different mechanisms for CH 3 + , CH 3 Br + and Br + ion formation, depending on the excitation energy and resonant Rydberg states. 1,2 These are summarized in Fig. 1 and Table 1 in terms of the number of photons (1–4) required prior to fragmentation (1, 2, 3c, 3b and 4 in Fig. 1) or autoionization (3a in Fig. 1). Ion signals due to three-photon dissociation processes appear as sharp peaks in REMPI spectra (Fig. 2a) but as broad features peaking at low kinetic energies in images and KER distributions (KERs). The sharp REMPI peaks are due to the two-photon level-to-level resonant excitations (resonant contribution) to the CH 3 Br Rydberg states (CH 3 Br**) and the broad KERs are due to energy redistribution among the molecule’s internal degrees of freedom prior to dissociation in metastable states. Ion signals due to one-photon dissociation non-resonant processes (non-resonant contributions), on the other hand, appear as a broad underlying continuum in the REMPI spectra (Fig. 2a) and as sharp peaks in KER distributions. a Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavı ´k, Iceland. E-mail: agust@hi.is; Web: https://notendur.hi.is/Bagust/; Tel: +354-525-4800 b Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, Vassilika Vouton, 71110 Heraklion, Greece. E-mail: sama@iesl.forth.gr; Fax: +30-2810-391305; Tel: +30-2810-391467 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c8cp06376a Received 12th October 2018, Accepted 18th April 2019 DOI: 10.1039/c8cp06376a rsc.li/pccp PCCP PAPER