Femtosecond XUV–IR induced photodynamics in the methyl iodide cation

Marta L Murillo-Sánchez,Rebeca De Nalda,Oleg Kornilov,Geert Reitsma,Jesús González-Vázquez,Luis Bañares,Sonia Marggi Poullain,Marc J J Vrakking,Fernando Martín,Pedro Fernández-Milán

doi:10.1088/1367-2630/ac0c9b

Abstract

The time-resolved photodynamics of the methyl iodide cation (CH3I+) are investigated by means of femtosecond XUV–IR pump–probe spectroscopy. A time-delay-compensated XUV monochromator is employed to isolate a specific harmonic, the 9th harmonic of the fundamental 800 nm (13.95 eV, 88.89 nm), which is used as a pump pulse to prepare the cation in several electronic states. A time-delayed IR probe pulse is used to probe the dissociative dynamics on the first excited state potential energy surface. Photoelectrons and photofragment ions— and I+—are detected by velocity map imaging. The experimental results are complemented with high level ab initio calculations for the potential energy curves of the electronic states of CH3I+ as well as with full dimension on-the-fly trajectory calculations on the first electronically excited state , considering the presence of the IR pulse. The and I+ pump–probe transients reflect the role of the IR pulse in controlling the photodynamics of CH3I+ in the state, mainly through the coupling to the ground state and to the excited state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of fragments after the cation is transferred to the ground states and an enhancement of I+ fragments by absorption of IR photons yielding dissociative photoionization.

Highlights

Methyl iodide has served for many years as a prototypical system for a variety of studies of photoinduced dynamics, from its photodissociation in the first absorption band to x-ray photoabsorption-induced charge transfer [6,7,8] among others
A voltage scheme is applied to the microchannel plates (MCP) detector to select specific fragment ions based on their time-of-flight (TOF)
The bands observed in the photoelectron spectrum (PES) are assigned to the different electronic states in which the CH3I+ is prepared after photoionization at 13.95 eV

Summary

Introduction

Methyl iodide has served for many years as a prototypical system for a variety of studies of photoinduced dynamics, from its photodissociation in the first absorption band (see, for instance, references [1,2,3,4,5]) to x-ray photoabsorption-induced charge transfer [6,7,8] among others. Its photoionization into CH3I+ and the subsequent dissociation has been actively investigated in particular employing multiphoton strong-field excitation [9,10,11,12,13,14,15,16,17,18,19,20] In such studies, intense IR femtosecond pulses have been often used to ionize the molecule leading eventually to dissociative photoionization (DPI) as well as to Coulomb explosion (CE) producing charged methyl and iodine fragments. Intense IR femtosecond pulses have been often used to ionize the molecule leading eventually to dissociative photoionization (DPI) as well as to Coulomb explosion (CE) producing charged methyl and iodine fragments Such an approach leads to a large number of reaction pathways and fragments, and to a challenging interpretation. The present experiments allow us to largely reduce the number of opened dissociation channels and to get a deeper understanding of the induced photodynamics of CH3I+

Methods

Results

Conclusion