Vibrational Coupling Pathways in Methanol As Revealed by Coherence-Converted Population Transfer Fourier Transform Microwave Infrared Double-Resonance Spectroscopy

Abstract

Coherence-converted population transfer infrared-microwave double-resonance spectroscopy is used to record the infrared spectra of jet-cooled CH(3)OH and CH(3)OD. Population transfer induced by a pulsed IR laser is detected by Fourier transform microwave spectroscopy background-free using a two-MW pulse sequence. The observed spectrum of CH(3)OH in the nu(3) symmetric CH stretch region contains 12 interacting vibrational bands, whereas in CH(3)OD, only one vibrational band is observed in the same interval (2750-2900 cm(-1)). The bright state, responsible for the transitions observed in this region, is not just nu(3) but also contains an admixture of the binary CH bending combinations, particularly 2nu(5). The lack of interacting bands in CH(3)OD confirms that in CH(3)OH the binary combinations of the OH bend (nu(6)) and a CH bend (nu(4), nu(5), nu(10)) act as doorway states linking the bright state to higher order combination vibrations involving torsional excitation. A time-dependent interpretation of the frequency-resolved spectra reveals a fast (approximately 200 fs) initial decay of the bright state followed by a slower (1-2 ps) redistribution among the lower frequency modes.

Millimeter-wave optical double resonance schemes for rapid assignment of perturbed spectra, with applications to the C̃1B2 state of SO2

“…We have also made a millimeter-wave implementation of the background free coherence-converted population transfer (CCPT) technique. [20][21][22] We find that CCPT provides an excellent improvement in signal-to-noise over traditional population labeling schemes, and is a promising alternative to polarization-based background-free double-resonance methods. 23 However, there are challenges to implementing the technique in our current experimental configuration, where the coherent experiment must be completed in a time window that is short compared to the millimeter-wave Doppler lifetime of <2 µs.…”

“…The technique enables rapid spectral acquisition and assignment of congested regions. Our millimeter-wave implementation of the background-free CCPT scheme, originally developed by Twagirayezu and coworkers, [20][21][22] significantly increases the mmODR sensitivity and allows us to observe weak transitions to levels that perturb theC(B a 1 ) level of SO 2 . Due to the relatively short Dopplerdephasing coherence time, we had difficulty implementing a multiplexed CCPT scheme.…”

“…[20][21][22] The technique is made possible by taking advantage of phase-coherent excitation and detection of the microwave FID. CCPT is a three-pulse sequence.…”

Coherence-converted population transfer FTMW-IR double resonance spectroscopy of CH3OD in the OD-stretch region

“…The technique, which is performed in a compact FTMW spectrometer [28] based on the Balle-Flygare design, [29] has been described in a previous paper [30], so only a brief summary is given here. An infrared (IR) laser pulse (Continuum Mirage OPO/OPA, $10 ns duration) and a sequence of two microwave (MW) pulses (each 500 ns) interact with jet-cooled CH 3 OD molecules.…”

Nonlinear two-dimensional terahertz photon echo and rotational spectroscopy in the gas phase

“…Strong optical fields have been used to drive high-order rotational coherences and to populate high-lying rotational levels (13), to control molecular alignment dynamics (14), and to induce rotational alignment echoes (15,16). Strong microwave fields can drive high-order rotational responses and have been used for double-resonance and 2D rotational spectroscopies (17)(18)(19)(20)(21)(22), mostly at low rotational temperatures. In recent work, we demonstrated second-order interactions between THz frequency fields and molecular rotations, measuring net molecular dipole orientation, manipulating two-quantum coherences (2QCs), and observing rotational populations and their superradiant decays (23)(24)(25).…”