Channel-selective analysis of light-induced wave packet dynamics in molecules

Abstract

Many light-induced molecular processes, including photosynthesis, biosynthesis of vitamin D, or isomerization in retinal chromophore, are essential for human existence on Earth. A traditional approach for studying such photochemical and photobiological reactions focuses on the initial molecular structure and the final products formed during these processes. However, this point of view does not provide information on the short-lived, transient molecular configurations, which often determine both the mechanisms and the outcome of the reaction. A particular challenge here is that light-induced changes in molecular structure typically occur on a very fast time scale, determined by the motion of individual atoms. Hence, with the development of femtosecond lasers and modern molecular imaging techniques, time-resolved studies of ultrafast light-induced dynamics have become an increasingly important topic in molecular physics and photochemistry. To capture ultrafast molecular dynamics in the time domain, femtosecond lasers are often used in a pump-probe scheme, where the pump pulse initiates the dynamics of interest and the probe pulse interrogates the evolving system at different times. Here, the probe step needs to employ an experimental technique that can provide information on evolving molecular structure. One of the promising approaches to address this goal is the so-called Coulomb Explosion Imaging (CEI), which relies on rapid multiple ionization and subsequent fragmentation of the target molecule. The information on molecular structure at the instant of laser-molecular interaction is then encoded in the momenta of the created positively charged ions, which can be measured as a function of time delay between pump and probe pulses. This approach can be efficiently combined with time-resolved ion mass spectrometry and Fourier analysis of different delay-dependent observables, including ion yields, their kinetic energies, or angular distributions, in particular, if COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS) is used to characterize the created ionic fragments. The thesis focuses on two major aspects of light-induced molecular dynamics studies. The first part of the thesis described the commissioning of a new COLTRIMS apparatus coupled with a newly installed 100 kHz femtosecond laser system at the James R. Macdonald Laboratory (JRML). This instrument is planned to be used in various pump-probe experiments at JRML, particularly focusing on CEI studies of photochemical processes. The corresponding section of the thesis briefly outlines the operational principle of COLTRIMS and describes essential components of the newly commissioned setup. This part also includes some preliminary experimental results on laser-induced fragmentation of chloroiodomethane (CH₂ClI) molecules obtained with the new apparatus. The second part of the thesis focuses on the study of strong-field-induced molecular wave packet dynamics in sulfur dioxide (SO₂) molecules. The experiments described in this section were conducted using an older COLTRIMS instrument coupled with two different femtosecond laser systems, operating at 3 kHz and 10 kHz repetition rates. In these experiments, an intense near- infrared (NIR,800 nm) or visible (400nm) pump pulse was used to trigger wave packet dynamics in SO₂ and SO₂⁺, which were probed by a second, more intense NIR pulse that further ionized and/or dissociated the molecule. The information on the time evolution of the created molecular wave packets was deduced by analyzing the delay-dependent yields of several singly, doubly, and triply charged final states of the molecule, as well as the kinetic energies and angular distribution of resulting ionic fragments. For either pump pulse used, we observed clear signatures of both vibrational and rotational dynamics. For the experiments employing the NIR pump pulse, where the delay between the pump and probe pulses ranged from 0 to 2 ps, channel-selective Fourier analysis revealed signatures of bending vibration in both ionic and neutral states of the SO₂ molecules. Fast Fourier Transform (FFT) spectra for all channels showed prominent peaks at the ionic bending vibrational frequency (~400 𝑐𝑚⁻¹), while some channels also displayed a weaker but clearer peak around 520 𝑐𝑚⁻¹, reflecting bending vibrations in the ground electronic state of the neutral SO₂ molecule. To reveal the initial direction of the wave packet motion and to characterize the corresponding wave packet phase in both ionic and neutral ground states, we performed the inverse Fast Fourier Transform (IFFT), selecting the corresponding frequency in the FFT spectra. For the experiments with the 400 nm pump pulse, observed vibrational dynamics have been dominated by the ionic state bending vibrations, reflected by the pronounced FFT peak around 400 𝑐𝑚⁻¹. In addition, a weak feature corresponding to the symmetric stretching mode at 1058 𝑐𝑚⁻¹ has been observed in the yield of (S⁺, O⁺, O⁺) coincidence channel. No signature of the neutral-state vibrational dynamics has been observed when using the 400 nm pump. For both pump frequencies used, we observed weak but clear impulsive alignment of the molecules by the pump laser pulse, reflected in the time evolution of the angular distributions of the fragments with respect to (linear) laser polarization. While in both cases the initial alignment deteriorated because of the rotational wave packet dephasing, the time scale of this dephasing was found to be significantly longer for the 400 nm pump pulse (~1.5 ps vs. ~ 0.5 ps for 800 nm pump), most likely because of the narrower distribution of the populated rotational states. In this experiment, where the pump-probe delay window was extended to 20 ps, we also observed a fractional revival of the rotational wave packet. Finally, for both pump wavelengths, we observed a clear decrease in the amplitude of the oscillations with 400 𝑐𝑚⁻¹ frequency at larger delays. For the experiments with the 400 nm pump, where a longer delay window was used, this oscillation was found to be completely suppressed after a few ps. However, for the delay-dependent yield of the doubly charged molecules (SO₂⁺⁺), we found that this oscillation is partially restored after ~9 ps, clearly visualized by the reappearance of the peak around 400 𝑐𝑚⁻¹ in the sliding-window FFT. A similar reappearance has not been found on any other channel so far. While the exact origin of this behavior and its channel specificity remain unclear, this could be a signature of the vibrational wave packet dephasing and subsequent revival - the phenomenon that is well understood in diatomic molecules but is much less studied in larger systems with an additional degree of freedom. Overall, the work reported in this thesis provides new insights into laser-induced dynamics in triatomic molecules and highlights the power of time-resolved ion spectroscopy, CEI, and channel-selective Fourier analysis for studies of ultrafast molecular processes. With the newly commissioned COLTRIMS setup in combination with the 100 kHz repetition rate of the new JRML laser, the experimental approach used in this work can be efficiently applied to map many light-induced reactions in more complex, chemically relevant systems.