S. Sunil Kumar

The absolute photodetachment cross-section characterizes the photostability of atomic and molecular anions against photodestruction by neutralization. The measurement of this quantity has been reported only for atomic and simple molecular ions. In 2006, Wester's group introduced a novel ion-trap-based technique to measure the absolute photodetachment cross-section [S. Trippel et al., Phys. Rev. Lett. 97, 193003 (2006)] of OH^-. In the present work, we propose a novel methodology to streamline this technique to reduce the measurement time by several orders of magnitude by combining a single experimental rate measurement with a simulated column density distribution of the trapped ions. We validated our approach by reproducing the cross-section reported for OH^- at 632.8 nm. Using this technique, we report the first such measurement for a molecule of biological interest, deprotonated indole, at a laser wavelength of 403 nm. The proposed scheme is anticipated to have a significant and transformative impact on the development of a comprehensive database for photodetachment and photodissociation cross-sections of molecular ions. Furthermore, these measurements have the potential to drive the development of cutting-edge computational codes for cross-section calculations, enabling an unprecedentedly detailed understanding of electron dynamics in large molecules and the light-matter interaction.

This review presents the Laser-Induced Fluorescence Spectroscopy (LIFS) of trapped gas-phase molecular ions. A brief description of the theory and experimental approaches involved in fluorescence spectroscopy, together with state-of-the-art LIFS experiments employing ion traps, is presented. Quadrupole ion traps are commonly used for spatial confinement of ions. One of the main challenges involved in such experiments is poor Signal-to-Noise Ratio (SNR) arising due to weak gas-phase fluorescence emission, high background noise, and small solid angle for the fluorescence collection optics. The experimental approaches based on the integrated high-finesse optical cavities provide a better (typically an order of magnitude more) SNR in the detected fluorescence than the single-pass detection schemes. Another key to improving the SNR is to exploit the maximum solid angle of light collection by choosing high numerical aperture (NA) collection optics. The latter part of the review summarises the current state-of-the-art intrinsic fluorescence measurement techniques employed for gas-phase studies. Also, the scope of these recent advances in LIFS instrumentation for detailed spectral characterisation of a fluorophore of weak gas-phase fluorescence emission is discussed, considering fluorescein as one example.