  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, $\nu_1+\nu_2+\nu_3+\nu_4^1+\nu_5^{-1}$ 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)
$\text{cm}^{-1}$, the rotational parameter $B$ was 1.162222 $\text{cm}^{-1}$,
and the quartic centrifugal distortion parameter $D$ was 3.998(62)$\times
10^{-6} \text{cm}^{-1}$, where the numbers in the parenthesis are one-standard
errors in the least significant digits.

Chemical Physics;Instrumentation and Detectors;Optics