Abstract
The predissociation dynamics of the 6s (B2E) Rydberg state of gas-phase CH3I were investigated by time-resolved Coulomb-explosion imaging using extreme ultraviolet (XUV) free-electron laser pulses. Inner-shell ionization at the iodine 4d edge was utilized to provide a site-specific probe of the ensuing dynamics. The combination of a velocity-map imaging (VMI) spectrometer coupled with the pixel imaging mass spectrometry (PImMS) camera permitted three-dimensional ionic fragment momenta to be recorded simultaneously for a wide range of iodine charge states. In accord with previous studies, initial excitation at 201.2 nm results in internal conversion and subsequent dissociation on the lower-lying A-state surface on a picosecond time scale. Examination of the time-dependent yield of low kinetic energy iodine fragments yields mechanistic insights into the predissociation and subsequent charge transfer following multiple ionization of the iodine products. The effect of charge transfer was observed through differing delay-dependencies of the various iodine charge states, from which critical internuclear distances for charge transfer could be inferred and compared to a classical over-the-barrier model. Time-dependent photofragment angular anisotropy parameters were extracted from the central slice of the Newton sphere, without Abel inversion, and highlight the effect of rotation of the parent molecule before dissociation, as observed in previous works. Our results demonstrate the ability to perform three-dimensional ion imaging at high event rates and showcase the potential benefits of this approach, particularly in relation to further time-resolved studies at free-electron laser facilities.
Highlights
Over several decades, methyl iodide (CH3I) has served as the benchmark system for photodissociation dynamics in polyatomic molecules both experimentally and theoretically [1,2,3,4,5,6,7]
The presence of iodine charge states beyond I3+ is a clear indication of multiphoton ionization initiated by the free-electron lasers (FELs) pulse
At our employed FEL photon energy of 95 eV, the ensuing C–I bond cleavage is probed in a site-specific manner due to the high absorption cross section of the iodine atom at this energy
Summary
Methyl iodide (CH3I) has served as the benchmark system for photodissociation dynamics in polyatomic molecules both experimentally and theoretically [1,2,3,4,5,6,7]. The C–I bond cleavage that arises from excitation of the first absorption band (A-band) constitutes one of the most studied processes in molecular photodissociation. The A-band absorption spectrum exhibits a broad structureless peak which gives rise to fast ballistic dissociation and results in population inversion between the spin–orbit excited, I∗(2P1/2), and ground, I(2P3/2), electronic states of the iodine atom [1, 2]. Numerous experimental groups have explored inner-shell excitation or ionization at the iodine atomic site to track the ensuing photodissociation dynamics following A-band excitation using both free-electron lasers (FELs) [9,10,11,12] and high harmonic generation sources [13, 14]. Iodine-containing hydrocarbons are an exemplary choice for performing extreme ultraviolet (XUV) time-resolved studies due to the I 4d5/2,3/2 → f shape resonances (60–140 eV) [15, 16], which result in the photoabsorption being dominated by the I atom and, the ability to excite or probe site-selectively
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