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Accurate photodissociation cross sections have been computed for transitions from the X $^1Sigma^+$ ground electronic state of CS to six low-lying excited electronic states. New ab initio potential curves and transition dipole moment functions have been obtained for these computations using the multi-reference configuration interaction approach with the Davidson correction (MRCI+Q) and aug-cc-pV6Z basis sets. State-resolved cross sections have been computed for transitions from nearly the full range of rovibrational levels of the X $^1Sigma^+$ state and for photon wavelengths ranging from 500 $text{AA}$ to threshold. Destruction of CS via predissociation in highly excited electronic states originating from the rovibrational ground state is found to be unimportant. Photodissociation cross sections are presented for temperatures in the range between 1000 and 10,000 K, where a Boltzmann distribution of initial rovibrational levels is assumed. Applications of the current computations to various astrophysical environments are briefly discussed focusing on photodissociation rates due to the standard interstellar and blackbody radiation fields.
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Photodissociation by ultraviolet radiation is the key destruction pathway for CS in photon-dominated regions, such as diffuse clouds. However, the large uncertainties of photodissociation cross sections and rates of CS, resulting from a lack of both laboratory experiments and theoretical calculations, limit the accuracy of calculated abundances of S-bearing molecules by modern astrochemical models. Here we show a detailed textit{ab initio} study of CS photodissociation. Accurate potential energy curves of CS electronic states were obtained by choosing an active space CAS(8,10) in MRCI+Q/aug-cc-pV(5+d)Z calculation with additional diffuse functions, with a focus on the (B) and (C,^1Sigma^+) states. Cross sections for both direct photodissociation and predissociation from the vibronic ground state were calculated by applying the coupled-channel method. We found that the (C-X) ((0-0)) transition has extremely strong absorption due to a large transition dipole moment in the Franck-Condon region and the upper state is resonant with several triplet states via spin-orbit couplings, resulting in predissociation to the main atomic products C ((^3P)) and S ((^1D)). Our new calculations show the photodissociation rate under the standard interstellar radiation field is (2.9ee{-9}),s(^{-1}), with a 57% contribution from (C-X) ((0-0)) transition. This value is larger than that adopted by the Leiden photodissociation and photoionization database by a factor of 3.0. Our accurate textit{ab initio} calculations will allow more secure determination of S-bearing molecules in astrochemical models.
In this work we discuss the rotational structure of Rydberg molecules. We calculate the complete wave function in a laboratory fixed frame and derive the transition matrix elements for the pho- toassociation of free ground state atoms. We discuss the implications for the excitation of different rotational states as well as the shape of the angular nuclear wave function. We find a rather com- plex shape and unintuitive coupling strengths, depending on the angular momenta coupling that are relevant for the states. This work explains the different steps to calculate the wave functions and the transition matrix elements in a way, that they can be directly transferred to different molecular states, atomic species or molecular coupling cases.
A calculation of dynamic polarizabilities of rovibrational states with vibrational quantum number $v=0-7$ and rotational quantum number $J=0,1$ in the 1s$sigma_g$ ground-state potential of HD$^+$ is presented. Polarizability contributions by transitions involving other 1s$sigma_g$ rovibrational states are explicitly calculated, whereas contributions by electronic transitions are treated quasi-statically and partially derived from existing data [R.E. Moss and L. Valenzano, textit{Molec. Phys.}, 2002, textbf{100}, 1527]. Our model is valid for wavelengths $>4~mu$m and is used to to assess level shifts due to the blackbody radiation (BBR) electric field encountered in experimental high-resolution laser spectroscopy of trapped HD$^+$ ions. Polarizabilities of 1s$sigma_g$ rovibrational states obtained here agree with available existing accurate textit{ab initio} results. It is shown that the Stark effect due to BBR is dynamic and cannot be treated quasi-statically, as is often done in the case of atomic ions. Furthermore it is pointed out that the dynamic Stark shifts have tensorial character and depend strongly on the polarization state of the electric field. Numerical results of BBR-induced Stark shifts are presented, showing that Lamb-Dicke spectroscopy of narrow vibrational optical lines ($sim 10$ Hz natural linewidth) in HD$^+$ will become affected by BBR shifts only at the $10^{-16}$ level.
We demonstrate rotational and vibrational cooling of cesium dimers by optical pumping techniques. We use two laser sources exciting all the populated rovibrational states, except a target state that thus behaves like a dark state where molecules pile up thanks to absorption-spontaneous emission cycles. We are able to accumulate photoassociated cold Cs2 molecules in their absolute ground state (v = 0, J = 0) with up to 40% efficiency. Given its simplicity, the method could be extended to other molecules and molecular beams. It also opens up general perspectives in laser cooling the external degrees of freedom of molecules.
Cooling the rotation and the vibration of molecules by broadband light sources was possible for trapped molecular ions or ultracold molecules. Because of a low power spectral density, the cooling timescale has never fell below than a few milliseconds. Here we report on rotational and vibrational cooling of a supersonic beam of barium monofluoride molecules in less than 440 $mu$s. Vibrational cooling was optimized by enhancing the spectral power density of a semiconductor light source at the underlying molecular transitions allowing us to transfer all the populations of $v=1-3$ into the vibrational ground state ($v=0$). Rotational cooling, that requires an efficient vibrational pumping, was then achieved. According to a Boltzmann fit, the rotation temperature was reduced by almost a factor of 10. In this fashion, the population of the lowest rotational levels increased by more than one order of magnitude.
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