title: Double resonant absorption measurement of acetylene symmetric vibrational states probed with cavity ring down spectroscopy.
abstract: A novel mid-infrared/near-infrared double resonant absorption setup for studying infrared-inactive vibrational states is presented. A strong vibrational transition in the mid-infrared region is excited using an idler beam from a singly resonant continuous-wave optical parametric oscillator, to populate an intermediate vibrational state. High output power of the optical parametric oscillator and the strength of the mid-infrared transition result in efficient population transfer to the intermediate state, which allows measuring secondary transitions from this state with a high signal-to-noise ratio. A secondary, near-infrared transition from the intermediate state is probed using cavity ring-down spectroscopy, which provides high sensitivity in this wavelength region. Due to the narrow linewidths of the excitation sources, the rovibrational lines of the secondary transition are measured with sub-Doppler resolution. The setup is used to access a previously unreported symmetric vibrational state of acetylene, ν1+ν2+ν3+ν4 (1)+ν5 (-1) in the normal mode notation. Single-photon transitions to this state from the vibrational ground state are forbidden. Ten lines of the newly measured state are observed and fitted with the linear least-squares method to extract the band parameters. The vibrational term value was measured to be at 9775.0018(45) cm(-1), the rotational parameter B was 1.162 222(37) cm(-1), and the quartic centrifugal distortion parameter D was 3.998(62) × 10(-6) cm(-1), where the numbers in the parenthesis are one-standard errors in the least significant digits.
authors.affiliation: Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland.
authors.forename: J
authors.initials: J
authors.lastname: Karhu
authors.affiliation: Van Swinderen Institute for Particle Physics and Gravity, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands.
authors.forename: J
authors.initials: J
authors.lastname: Nauta
authors.affiliation: Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland.
authors.forename: M
authors.initials: M
authors.lastname: Vainio
authors.affiliation: Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland.
authors.forename: M
authors.initials: M
authors.lastname: Metsälä
authors.affiliation: Van Swinderen Institute for Particle Physics and Gravity, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands.
authors.forename: S
authors.initials: S
authors.lastname: Hoekstra
authors.affiliation: Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland.
authors.forename: L
authors.initials: L
authors.lastname: Halonen