Ab Initio Study of Ground-state CS Photodissociation via Highly Excited Electronic States

Abstract

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 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 1 Σ + 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 ( 3 P ) and S ( 1 D). Our new calculations show the photodissociation rate under the standard interstellar radiation field is 2.9 × 10 −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 ab initio calculations will allow more secure determination of S-bearing molecules in astrochemical models.

Multireference configuration interaction study of the predissociation of C2 via its F1Π u state

“…As discussed later, the additional diffuse functions are important for obtaining accurate electronic energies for Rydberg states. In total, the basis set comprises 270 orbitals, with (50,33,33,19,50,33,33,19) symmetry-adapted functions in D 2h . Tests were also performed using additional Dunning's augmented core-valence basis sets aug-cc-pCV5Z and augcc-pCV6Z [58,59]; as expected, for Rydberg states these basis sets had minimal effect on the calculated energy but increased the calculation time by a factor of about 2.5.…”

“…Then, least-squares fitting programs are used to optimize the model parameters, which include potential energy curves, transition dipole moments and couplings between states, to match the calculated cross sections to experimentally measured cross sections and linewidths. We successfully employed this method, using ab initio model parameters computed at the MRCI level to study the predissociation of CS 1 Σ + states [50]. In this study, the predissociation diabatic coupled channel model of C 2 is built by including PECs of excited states of interest and the appropriate SOC values and non-adiabatic couplings among them.…”

“…Recently, we have investigated the photodissociation of CS through high-energy Rydberg states using ab initio MRCI methods with a reference space generated by the complete active space selfconsistent field (CASSCF) technique [50]. By including several Rydberg molecular orbitals into the active space of the CASSCF reference and adding extra diffuse functions to the basis set, the adiabatic PECs of several Rydberg states were obtained successfully.…”

Improving Excited-State Potential Energy Surfaces via Optimal Orbital Shapes

“…Let us begin with the relatively simple example of C–S bond photodissociation, which can be important in astrochemistry, biomedicine, and catalysis . For C–S single bond dissociation, nonradiative internal conversion between the first excited state 1 ππ* and the dissociative 1 πσ* state is key .…”

Dramatic effect of the nature of R on the intrinsic acidity and basicity of potential astrochemical R–C≡COH and R–C≡CSH compounds

“…Many of the compounds detected in the interstellar medium are hard to be synthesized in the laboratory, and therefore, the information we have on their structure, properties and reactivity is fragmentary, and much of it comes from theoretical approaches. Indeed, mainly along the last decade, a lot of information on the reactivity of many potential astrochemical compounds came from high-level ab initio calculations [1][2][3][4][5][6][7][8]. In a similar way, it was possible to determine the structure of new or potential astrochemical compounds [9][10][11][12] or predict their existence and/ or their spectroscopic properties [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26], or to explore their behavior in water or ice [27][28][29].…”