The acetylene ground state saga

On the 12C2H2 near-infrared spectrum: absolute transition frequencies and an improved spectroscopic network at the kHz accuracy level

Lamb dips of twenty lines in the P, Q, and R branches of the ν1 + ν3 + ν41 vibrational band of 12C2H2, in the spectral window of 7125-7230 cm-1, have been measured using an upgraded comb-calibrated frequency-stabilized cavity ring-down spectrometer, designed for extensive sub-Doppler measurements. Due to the large number of carefully executed Lamb-dip experiments, and to the extrapolation of absolute frequencies to zero pressure in each case, the combined average uncertainty of the measured line-center positions is 15 kHz (5 × 10-7 cm-1) with a 2-σ confidence level. Selection of the twenty lines was based on the theory of spectroscopic networks (SN), ensuring that a large number of transitions, measured previously by precision-spectroscopy investigations, could be connected to the para and ortho principal components of the SN of 12C2H2. The assembled SN contains 331 highly precise transitions, 119 and 121 of which are in the ortho and para principal components, respectively, while the rest remain in floating components. The para- and ortho-12C2H2 energy-level lists, determined during the present study, contain 82 and 80 entries, respectively, with an accuracy similar to that of the lines. Based on the newly assembled lists of para- and ortho-12C2H2 empirical energy levels, a line list, called TenkHz, has been generated. The TenkHz line list contains 282 entries in the spectral range of 5898.97-7258.87 cm-1; thus far, only 149 of them have been measured directly via precision spectroscopy. The TenkHz line list includes 35 intense lines that are missing in the HITRAN2020 database.

Laser control of a dark vibrational state of acetylene in the gas phase—Fourier transform pulse shaping constraints and effects of decoherence

We propose a methodology to tackle the laser control of a non-stationary dark ro-vibrational state of acetylene (C2H2), given realistic experimental limitations in the 7.7 μm (1300 cm−1) region. Simulations are performed using the Lindblad master equation, where the so-called Lindblad parameters are used to describe the effect of the environment in the dilute gas phase. A phenomenological representation of the parameters is used, and they are extracted from high-resolution spectroscopy line broadening data. An effective Hamiltonian is used for the description of the system down to the rotational level close to experimental accuracy. The quality of both the Hamiltonian and Lindblad parameters is assessed by a comparison of a calculated infrared spectrum with the available experimental data. A single shaped laser pulse is used to perform the control, where elements of optics and pulse shaping using masks are introduced with emphasis on experimental limitations. The optimization procedure, based on gradients, explicitly takes into account the experimental constraints. Control performances are reported for shaping masks of increasing complexity. Although modest performances are obtained, mainly due to the strong pulse shaping constraints, we gain insights into the control mechanism. This work is the first step toward the conception of a realistic experiment that will allow for population characterization and manipulation of a non-stationary vibrational “dark” state. Effects of the collisions on the laser control in the dilute gas phase, leading to decoherence in the molecular system, are clearly shown.

Lindblad parameters from high resolution spectroscopy to describe collision-induced rovibrational decoherence in the gas phase-Application to acetylene

Within the framework of the Lindblad master equation, we propose a general methodology to describe the effects of the environment on a system in the dilute gas phase. The phenomenological parameters characterizing the transitions between rovibrational states of the system induced by collisions can be extracted from experimental transition kinetic constants, relying on energy gap fitting laws. As the availability of these kinds of experimental data can be limited, this work relied on experimental line broadening coefficients, however still using energy gap fitting laws. The 3 μm infrared spectral range of acetylene was chosen to illustrate the proposed approach. The method shows fair agreement with available experimental data while being computationally inexpensive. The results are discussed in the context of state laser quantum control.

Large amplitude motion of the acetylene molecule within acetylene-neon complexes hosted in helium droplets

Superfluid helium droplets provide an ideal environment for spectroscopic studies with rotational resolution. Nevertheless, the molecular rotation is hindered because the embedded molecules are surrounded by a non-superfluid component. The present work explores the dynamical role of this component in the hindered rotation of C2H2 within the C2H2-Ne complex. A HENDI experiment was built and near-infrared spectroscopy of C2H2-Ne and C2H2 was performed in the spectral region overlapping the ν3/ν2 + ν4 + ν5 Fermi-type resonance of C2H2. The comparison between measured and simulated spectra helped to address the above issue.

Comb-assisted cavity ring-down spectroscopy of a buffer-gas-cooled molecular beam

We demonstrate continuous-wave cavity ring-down spectroscopy of a partially hydrodynamic molecular beam emerging from a buffer-gas-cooling source.

The 4ν(CH) overtone of 12C2H2: sub-MHz precision spectrum reveals perturbations

The third CH stretching vibration overtone (4ν(CH)) of the acetylene molecule has been a prototype for intra-molecular dynamics studies. Using a sensitive cavity ring-down spectrometer calibrated with precise atomic transitions, the absolute line frequencies of 50 lines of this band have been determined with sub-MHz accuracy, or relatively 2 × 10(-9). The accuracy is also confirmed by the combination differences between the transitions sharing the same upper level. The improved accuracy, two orders of magnitude better than previous studies, allows us to reveal finer ro-vibrational couplings. Fitting of the rotational energies indicates that the J-dependent interactions take place after J > 7. The precise line positions present useful confinements to the models of the intra-molecular interactions of the acetylene molecule.

Vibrational modes of the vinyl and deuterated vinyl radicals

Following the initial report of the detection of fundamental transitions of all nine vibrational modes of the vinyl radical [Letendre , L. ; Liu , D.-K. ; Pibel , C. D. ; Halpern , J. B. ; Dai , H.-L. J. Chem. Phys. 2000 , 112 , 9209] by time-resolved IR emission spectroscopy, we have re-examined the assignments of the vibrational modes through isotope substitution. Precursor molecules vinyl chloride-d3, vinyl bromide-d3, and 1,3-butadiene-d6 are used for generating vibrationally excited vinyl-d3 through 193 nm photolysis. The nondeuterated versions of these molecules along with vinyl iodide and methyl vinyl ketone are used as precursors for the production of vinyl-h3. IR emission following the 193 nm photolysis laser pulse is recorded with nanosecond time and approximately 8 cm(-1) frequency resolution. A room-temperature acetylene gas cell is used as a filter to remove the fundamental transitions of acetylene, a photolysis product, in order to reduce the complexity of the emission spectra. Two-dimensional cross-spectra correlation analysis is used to identify the emission bands from the same emitting species and improve the S/N of the emission spectra. Isotope substitution allows the identification of several low-frequency vibrational modes. For C2H3, the assigned modes are the nu4 (CC stretch) at 1595, nu5 (CH2 symmetric bend) at 1401, nu6 (CH2 asymmetric + alpha-CH bend) at 1074, nu8 (CH2 + alpha-CH symmetric out-of-plane (oop) bend) at 944, and nu9 (CH2 + alpha-CH asymmetric oop bend) at 897 cm(-1). For C2D3, the modes are the nu5 (CD2 symmetric bend) at 1060, nu6 (CD2 asymmetric + alpha-CD bend) at 820, and nu8 (CD2 + alpha-CD symmetric oop bend) at 728 cm(-1).