Mapping and controlling ultrafast dynamics of highly excited H2 molecules by VUV-IR pump-probe schemes

Abstract

We used ultrashort femtosecond vacuum ultraviolet (VUV) and infrared (IR) pulses in a pump-probe scheme to map the dynamics and nonequilibrium dissociation channels of excited neutral H 2 molecules. A nuclear wave packet is created in the B 1 + u state of the neutral H 2 molecule by absorption of the ninth harmonic of the driving infrared laser field. Due to the large stretching amplitude of the molecule excited in the B 1 + u electronic state, the effective H 2 + ionization potential changes significantly as the nuclear wave packet vibrates in the bound, highly electronically and vibrationally excited B potential-energy curve. We probed such dynamics by ionizing the excited neutral molecule using time-delayed VUV-or-IR radiation. We identified the nonequilibrium dissociation channels by utilizing three-dimensional momentum imaging of the ion fragments. We found that different dissociation channels can be controlled, to some extent, by changing the IR laser intensity and by choosing the wavelength of the probe laser light. Furthermore, we concluded that even in a benchmark molecular system such as H 2 *, the interpretation of the nonequilibrium multiphoton and multicolor ionization processes is still a challenging task, requiring intricate theoretical analysis.

H2: the benchmark molecule for ultrafast science and technologies

“…Thus, the second challenge is to find the appropriate detection mechanism to trace dynamics and to even image them. While techniques like laser-induced fluorescence [101] represent a great tool to trigger and detect molecular systems with transitions in the UV to the near infrared (NIR) range, one would need a pump wavelength of ∼100 nm [102] to reach the first excited state of the neutral H 2 molecule-a challenge even today as we will see later [103][104][105][106], but not accessible 20 years ago. The desire to resolve vibrational and electronic dynamics in H 2 has thus triggered two branches of development-shorter laser pulses, and advanced and creative imaging techniques, which will be introduced in the following.…”

Orientation-dependent dissociative ionization of H2 in strong elliptic laser fields: Modification of the release time through molecular orientation

“…The net-2ω channel yields higher kinetic energy than the 1ω dissociation. Here, we discuss only on the net-2ω channel as this channel is easy to distinguish regarding inter-channel mixing compared to the 1ω channel [21]. Fig.…”

The quantum chemistry of attosecond molecular science

“…Theory based on the spectral method has also given support to time‐resolved experiments that make use of an APT in combination with one or several weak IR fields, showing that the ratio between dissociative and nondissociative ionization can be manipulated in H 2 and D 2 by just varying the photon energy of the pulses.…”