Electronic and vibrational relaxation dynamics of NH3 Rydberg states probed by vacuum-ultraviolet time-resolved photoelectron imaging

Time-resolved dynamics of high-lying Rydberg states of ammonia (NH3) prepared by using a vacuum ultraviolet (VUV) pump (∼9.3 eV) and an ultraviolet (UV) probe (∼4.7 eV) pulse are reported using photoelectron imaging detection. After photoexcitation, two main features appear in the photoelectron spectrum with vertical binding energies of ∼1.8 eV and ∼3.2 eV and with distinctly different anisotropy parameters β of ∼1.3 and ∼0.7, respectively. This information allows the unambiguous assignment of the respective.

Many spectroscopic and theoretical studies have been devoted to ammonia (NH3). Studies which have focused on electronically excited NH3 have been mainly focused on the low-lying Rydberg states (∼6 eV) investigating their complex photodissociation dynamics. The few studies that have involved high-lying Rydberg states have been confined to static measurements because of a lack of vacuum ultraviolet (VUV) femtosecond laser sources.

In many biologically relevant molecules, such as amino acids, the sp3 hybridized nitrogen is the central functional unit in defining their photochemistry, e.g., through valence-Rydberg mixing that causes their surprising stability to UV light.10,11 Further under- standing of the mechanisms by which these nitrogen centers can efficiently redistribute absorbed UV light therefore has a natural imperative that invites further study. While there are certainly differences between the photoinduced molecular dynamics of C–N bonds and N–H bonds, the study of small molecules can shed light on the fundamental dynamics of moieties that could be clouded by increased complexity, in addition to having the benefit of being accessible to state-of-the-art quantum-chemical calculations without facing extreme computational costs.

NH3 makes an ideal candidate system as it is a small molecule with a sp3 hybridized nitrogen center. Moreover, hydrogen substitution leads to the creation of nitrogen containing biomolecules, and its spectroscopy can be interpreted in terms of simple symmetry arguments. Due to this, many studies have been devoted to the spectroscopy of NH3. The early works studying the photodissociation dynamics were pioneered by Ashfold and co-workers, who paved the way for most of the subsequent interest in the à state of NH3. Since then, many research groups contributed to this effort.15–19 However, these experiments were limited to static studies, due to the lack of ultrashort pulses in the UV/VUV domain.

Recently, table-top UV/VUV femtosecond sources emerged and have proven highly valuable for dynamical studies.20–26 The natural evolution of this experimental effort in understanding the photophysics of NH3 is then to use these femtosecond UV/VUV sources and investigate the time-resolved dynamics of the high-lying Rydberg states in NH3.

In the present study, the dynamics of the high-lying Rydberg states of NH3 are followed through time- and angle-resolved photoelectron spectroscopy. The states involved are fully characterized utilizing a time-resolved velocity-map-imaging (VMI) spectrometer with a VUV pump (133 nm) and a UV probe (266 nm). One of the strengths of VMI spectroscopy is that the technique allows one to capture the photoelectron spectrum while simultaneously obtaining the photoelectron angular distribution (PAD). This PAD is related to the orbital-angular-momentum character of the ejected photo- electron, which carries information about the corresponding character of the photoionized orbital. The time- and energy-dependencies of the PAD can assist the interpretation of the observed dynamics or even provide additional insights.

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