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

Abstract

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 ${\text{I}}^{-}\cdot {\text{H}}_2\text{O}$ 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 ${\text{I}}^{-}\cdot {\text{H}}_2\text{O}$ complex is monitored by recording the ${\text{I}}^{-}$ product yield by time-of-flight mass spectrometry as a function of laser wavelength. Very narrow ($\mathrm{\Delta }$v∼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.

GRAPHICAL ABSTRACT

KEYWORDS:

• Rovibrational spectroscopy
• cluster spectroscopy
• rotational constant
• tunable laser
• ion trap

Acknowledgements

M.A.J and ABM gratefully acknowledge the Department of Energy through the condensed phase and interfacial molecular science (CPIMS) programme under grants DE-SC0021012 and DE-SC0021081 for support of this work. We also thank Toptica Photonics, Inc., for the loan of the TOPO laser system used in this work. S.C.E. was supported by an AFOSR MURI grant 62742085-204669. M.H. was supported by the U.S. National Science Foundation funding CHE-1900532. We thank Thien Khuu and Abhijit Rana for their invaluable assistance in the operation of secular frequency mass selection in the Paul trap. We are especially grateful to Prof. Dr. Dieter Gerlich for his development of the powerful ion guiding and trapping methods central to this work, as well as his encouragement in our early efforts to apply them to the vibrational spectra of cold ions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Significance

We describe the integration of a tunable continuous wave infrared laser into a cryogenic ion spectroscopy experiment to measure the rovibrational spectrum of the ${\text{I}}^{-}\cdot {\text{H}}_2\text{O}$ complex in the OH stretching region. Significantly, analysis of the resulting spectra yields rotational structural constraints for the ${\text{I}}^{-}\cdot {\text{H}}_2\text{O}$ complex for the first time. Furthermore, we unexpectedly find that most of the trapped ion ensemble can be photodissociated upon excitation of a single rotational line. This useful characteristic of the experimental protocol enables high signal-to-noise in the photodissociation spectrum and is traced to rapid equilibration of the ground state rotational levels through buffer gas collisions.

Additional information

Funding

This work was supported by Air Force Office of Scientific Research: [Grant Number 62742085-204669]; the U.S. National Science Foundation: [Grant Number CHE-1900532]; U.S. Department of Energy: [Grant Number DE-SC0021012]; U.S. Department of Energy: [Grant Number DE-SC0021081].