High-resolution vibrational predissociation spectroscopy of I-·H2O by single-mode CW infrared excitation in a 3D cryogenic ion trap

• Published in: Molecular physics 2023-02, Vol.122 (1-2)
• Main Authors: Harville, Payten Anne, Edington, Sean, Moss, Olivia, Huang, Meng, McCoy, Anne, Johnson, Mark Albert
• Format: Article
• Language: English
• Subjects: ATOMIC AND MOLECULAR PHYSICS; cluster spectroscopy; ion trap; rotational constant; rovibrational spectroscopy; tunable laser
• ISSN: 0026-8976; 1362-3028

We describe the integration of a tunable, single-mode, continuous wave infrared laser into a cryogenic ion spectroscopy experiment to measure the rovibrational spectrum of the I-∙H2O complex in the OH stretching region. These upper levels lie about 300 cm-1 above the dissociation threshold. The measurements are carried out by loading the ions in a radiofrequency ion trap at 10 Hz and cooling them to 5 K with pulsed He buffer gas. IR photodissociation (PD) of the I-∙H2O complex is monitored by recording the I- product yield by time-of-flight mass spectrometry as a function of laser wavelength. Very narrow (Δν~75 MHz) rotational lines are observed throughout the spectrum, indicating long (ca. 2 ns) lifetimes for the excited metastable rovibrational levels. Rotational analysis of the band arising from the K”=1 to K’=2 transition of the free OH stretching fundamental yields the structure of the complex for the first time. Over 50% of the trapped ion ensemble in the trap can be photodissociated upon excitation of a single rotational line. This enables very high signal-to-noise in the PD spectrum, and is traced to a mechanism in which the ground state rotational levels are rapidly equilibrated by collisions with the buffer gas.