Cluster size resolving analysis of CH3F-(ortho-H2)n in solid para-hydrogen using FTIR absorption spectroscopy at 3 μm region

Full quantum calculation of the rovibrational states and intensities for a symmetric top-linear molecule dimer: Hamiltonian, basis set, and matrix elements

The rovibrational energy levels and intensities of the CH₃F-H₂ dimer have been obtained using our recent global intermolecular potential energy surface [X.-L. Zhang et al., J. Chem. Phys. 148, 124302 (2018)]. The Hamiltonian, basis set, and matrix elements are derived and given for a symmetric top-linear molecule complex. This approach to the generation of energy levels and wavefunctions can readily be utilized for studying the rovibrational spectra of other van der Waals complexes composed of a symmetric top molecule and a linear molecule, and may readily be extended to other complexes of nonlinear molecules and linear molecules. To confirm our method, the rovibrational levels of the H₂O-H₂ dimer have been computed and shown to be in good agreement with experiment and with previous theoretical results. The rovibrational Schrödinger equation has been solved using a Lanczos algorithm together with an uncoupled product basis set. As expected, dimers containing ortho-H₂ are more strongly bound than dimers containing para-H₂. Energies and wavefunctions of the discrete rovibrational levels of CH₃F-paraH₂ complexes obtained from the direct vibrationally averaged 5-dimensional potentials are in good agreement with the results of the reduced 3-dimensional adiabatic-hindered-rotor (AHR) approximation. Accurate calculations of the transition line strengths for the orthoCH₃F-paraH₂ complex are also carried out, and are consistent with results obtained using the AHR approximation. The microwave spectrum associated with the orthoCH₃F-orthoH₂ dimer has been predicted for the first time.

Analytic Morse/long-range potential energy surfaces and "adiabatic-hindered-rotor" treatment for a symmetric top-linear molecule dimer: A case study of CH3F-H2

A first effective six-dimensional ab initio potential energy surface (PES) for CH₃F-H₂ which explicitly includes the intramolecular Q₃ stretching normal mode of the CH₃F monomer is presented. The electronic structure computations have been carried out at the explicitly correlated coupled cluster level of theory [CCSD(T)-F12a] with an augmented correlation-consistent triple zeta basis set. Five-dimensional analytical intermolecular PESs for ν₃(CH₃F) = 0 and 1 are then obtained by fitting the vibrationally averaged potentials to the Morse/Long-Range (MLR) potential function form. The MLR function form is applied to the nonlinear molecule-linear molecule case for the first time. These fits to 25 015 points have root-mean-square deviations of 0.74 cm^-1 and 0.082 cm^-1 for interaction energies less than 0.0 cm^-1. Using the adiabatic hindered-rotor approximation, three-dimensional PESs for CH₃F-paraH₂ are generated from the 5D PESs over all possible orientations of the hydrogen monomer. The infrared and microwave spectra for CH₃F-paraH₂ dimer are predicted for the first time. These analytic PESs can be used for modeling the dynamical behavior in CH₃F-(H₂)_N clusters, including the possible appearance of microscopic superfluidity.

Experimental verification of the cluster model of CH3F-(ortho-H2)n in solid para-H2 by using mid-infrared pump-probe laser spectroscopy

The bleaching process in the C-F stretching mode (ν₃ band) of CH₃F-(ortho-H₂)_n [n = 0 and 1] clusters in solid para-H₂ was monitored using pump and probe laser spectroscopy on the C-H stretching mode (ν₁ and 2ν₅ bands). From an analysis of the depleted spectral profiles, the transition frequency and linewidth of each cluster were directly determined. The results agree with the values previously derived from a deconvolution analysis of the broadened ν₁/2ν₅ spectrum observed by FTIR spectroscopy. The complementary increase and decrease between the n = 0 and 1 components were also verified through monitoring the ν₁ and 2ν₅ bands, which suggests a closed system among the CH₃F-(ortho-H₂)_n clusters. These observations provide experimental verification of the CH₃F-(ortho-H₂)_n cluster model. On the other hand, a trial to observe the bleaching process by pumping the C-H stretching mode was not successful. This result may be important for understanding the dynamics of vibrational relaxation processes in CH₃F-(ortho-H₂)_n in solid para-H₂.