Terahertz-visible two-photon rotational spectroscopy of coldOD−

Abstract

We present a method to measure rotational transitions of molecular anions in the terahertz domain by sequential two-photon absorption. Ion excitation by bound-bound terahertz absorption is probed by absorption in the visible on a bound-free transition. The visible frequency is tuned to a state-selective photodetachment transition of the excited anions. This provides a terahertz action spectrum for just few hundred molecular ions. To demonstrate this we measure the two lowest rotational transitions, J =1←0 and J =2←1 of OD − anions in a cryogenic 22-pole trap. We obtain rotational transition frequencies of 598596.08(19) MHz for J =1←0 and 1196791.57(27) MHz for J =2←1 of OD − , in good agreement with their only previous measurement. This two-photon scheme opens up terahertz rovibrational spectroscopy for a range of molecular anions, in particular for polyatomic and cluster anions.

Rotational Spectroscopy of a Triatomic Molecular Anion

“…In this way we have identified ranges where the detachment rate depends on the thermal rotational excitation of the trapped ions. This is caused by photodetachment from rotationally excited states in the anion, as found previously for OH − [31,37]. The results of these measurements are described in detail in Ref.…”

“…2 ) is detected by extracting the ions from the trap and passing them onto a charge-integrating micro-channel plate detector. To reach a higher sensitivity compared to our previous experiment with OD − [31] the buffer gas density is reduced by a factor of 20. Thus, inelastic relaxation collisions with helium buffer gas atoms, which counteract the THz excitation, happen at a lower rate.…”

“…THz geometry, the transfer efficiency is less than 10% due to scattering and diffraction of the radiation with 0.2-3 mm wavelength at the 6.6 mm trap opening [31]. The THz frequency is continuously monitored with a high resolution wavemeter (HighFinesse WS-U).…”

“…Rotational state-dependent attachment of He atoms to cold molecules has been successfully applied to CD + [29], l-C 3 H + [9], CF + and NH 3 D + [30]. Terahertz-visible two-photon rotational spectroscopy is an alternative method that has been developed in our group to probe rotational transitions of molecular anions and this has been applied to OD − [31].…”

Effect of rotational excitation of H2 on isotopic exchange reaction with OD− at low temperatures

“…The importance of the OH − ion is underlined by a number of recent laboratory and theoretical studies of its destruction (Hlavenka et al 2009;), formation Plašil et al 2017), inelastic collisions ), or its rotational spectrum Matsushima et al 2006;Jusko et al 2014;Lee et al 2016). Additionally, the OH − anion has become a tool for thermometry of ions in ion traps (Otto et al 2013;) and storage rings (DESIREE, Schmidt et al 2017;CSR, Meyer et al 2017), based on threshold photodetachment spectroscopy.…”