Single Vibronic Level Emission Research Articles

Optical spectroscopy of the previously unobserved palladium monosulfide (PdS) radical.

The optical spectra of the palladium monosulfide (PdS) molecule in gas phase have been investigated for the first time through laser-induced fluorescence (LIF) and single-vibronic-level (SVL) emission spectroscopies. The I3Σ- - X3Σ- transition system containing 16 vibronic bands was identified in the LIF spectra, covering the energy range of 22 030-23 400cm-1. The spectra with rotational resolution allowed for the determination of the molecular constants in both the ground X and excited I electronic states, involving the spin-orbit splitting, rotational constant, vibrational frequency, and isotope shift. Isotopically resolved SVL emission spectra permitted the observation of the spin-orbit splitting, vibrational frequency, and vibrational isotope shift of the X3Σ-0+,1 and A3Π2,1,0-,0+ states as they transitioned from the excited I state to the vibrational levels of the X and A states. Ab initio calculations presented plenty of the Λ-S and Ω states of PdS below 28 000cm-1 and provided strong support for the assignments of the experimental observation.

Spectroscopic detection of the stannylidene (H2C=Sn and D2C=Sn) molecule in the gas phase.

The H2CSn and D2CSn molecules have been detected for the first time by laser-induced fluorescence (LIF) and emission spectroscopic techniques through the B̃1B2-X̃1A1 electronic transition in the 425-400nm region. These reactive species were prepared in a pulsed electric discharge jet using (CH3)4Sn or (CD3)4Sn diluted in high-pressure argon. Transitions to the electronic excited state of the jet-cooled molecules were probed with LIF, and the ground state and low-lying Ã1A2 state energy levels were measured from single vibronic level emission spectra. We supported the experimental studies by a variety of ab initio calculations that predicted the energies, geometries, and vibrational frequencies of the ground and lower excited electronic states. The spectroscopy of stannylidene (H2CSn) is in many aspects similar to that of silylidene (H2CSi) and germylidene (H2CGe).

Barely fluorescent molecules. I. Twin-discharge jet laser-induced fluorescence spectroscopy of HSnCl and DSnCl.

The divalent tin transient molecules HSnCl and DSnCl have been detected for the first time by laser-induced fluorescence (LIF) spectroscopy. HSnCl/DSnCl were produced in a twin-discharge jet using separate precursor streams of SnH4/SnD4 and the discharge products from HCl/DCl, both diluted in high pressure argon. The Ã1A″-X̃1A' spectrum of HSnCl consists of a single vibronic 00 0 band with a very short fluorescence lifetime (∼30ns). In contrast, the LIF spectrum of DSnCl exhibits three bands (00 0,20 1,and20 2), whose fluorescence lifetimes decrease from 393ns (00) to less than 10ns (22). Single vibronic level emission spectra have been recorded, providing information on all three vibrational modes in the ground state. Previous detailed ab initio studies indicate that these molecules dissociate into SnCl + H on the excited state potential surface and this is the cause of the short fluorescence lifetimes and breaking off of the fluorescence. It is fortunate that the HSnCl excited state zero-point level is still fluorescent or it would not be detectable by LIF spectroscopy.

Barely fluorescent molecules. II. Twin-discharge jet laser-induced fluorescence spectroscopy of HSnBr and DSnBr.

HSnBr and DSnBr have been detected for the first time by a combination of laser-induced fluorescence (LIF), fluorescence hole-burning, and wavelength resolved emission spectroscopies. The transient molecules were produced in a twin-discharge jet using separate precursor streams of SnH4/SnD4 and HBr/DBr, both diluted in high pressure argon. The Ã1A″-X̃1A' spectrum of HSnBr only consists of the 00 0 and 20 1 cold bands that show clearly resolved subband structure with fluorescence lifetimes varying from 526 to 162ns. The DSnBr LIF spectrum exhibits four bands (00 0, 20 1, 20 2, and 10 1) whose fluorescence lifetimes decrease from 525ns (00) to 175ns (11). Single vibronic level emission spectra have provided extensive information on the ground state vibrations, including all the anharmonicities and the harmonic frequencies. Fluorescence hole-burning experiments have shown that a few higher HSnBr nonfluorescent levels are very short-lived but still detectable. The ab initio studies of Tarroni and Clouthier [J. Chem. Phys. 156, 064304 (2022)] show that these molecules dissociate into SnBr + H on the excited state potential surface and this is the cause of the short fluorescence lifetimes and breaking off of the LIF spectra. HSnBr is a barely fluorescent molecule in the sense that only vibrational levels less than or equal to 317cm-1 in the excited state emit detectable photons down to the ground state.

Spin-orbit splittings in the low-lying states of MoO molecule

The laser-induced fluorescence (LIF) excitation spectra and single vibronic level (SVL) emission spectra of the jet-cooled molybdenum monoxide (MoO) molecule in the gas phase have been investigated in the range of 16,500–23,200 cm−1. Totally 11 rotationally resolved excitation spectra have been observed, and the rotational constants in 10 upper excited states and 4 lower spin-orbit components of the ground X5Π state have been obtained. In addition, 10 spin-orbit components in the ground X5Π state, the first and second excited states a3Σ− and b3∆, have been identified in the emission spectra. Ab initio calculations have been performed and used to identify the observed low-lying states.

Identification of the Jahn-Teller active trichlorosiloxy (SiCl3O) free radical in the gas phase.

The Ã2A1-X̃2E electronic transition of the jet-cooled trichlorosiloxy (SiCl3O) free radical has been observed for the first time in the 650-590 nm region by laser induced fluorescence (LIF) detection. The radical was produced by a pulsed electric discharge through a mixture of silicon tetrachloride and oxygen in high pressure argon at the exit of a pulsed molecular beam valve. The LIF spectrum shows low frequency intervals, which we assign as activity in the normally forbidden degenerate v5 ' and v6 ' modes, indicative of a significant Jahn-Teller effect in the ground state. Single vibronic level emission spectra show level dependent spin-orbit splittings in the ground state and Jahn-Teller predictable variations depending on which upper state level is pumped. The measured lower state energy levels have been fitted to a Jahn-Teller model that simultaneously includes spin-orbit coupling and linear and quadratic multimode coupling. In SiCl3O, the Jahn-Teller interaction predominates over spin-orbit effects.

Spin-orbit splitting in the ground state of tungsten monosulfide

The laser-induced fluorescence (LIF) excitation spectra and single vibronic level (SVL) emission spectra of tungsten monosulfide (WS) molecule have been recorded in the range of 13,100–16,500 cm−1 using laser ablation/reaction free-jet expansion. 13 vibronic bands are observed and grouped into 3 electronic band systems including the [15.30]1–X0+, [15.10]1–X0+, and [a + 13.12]1–X1 transitions. The first two transition progressions are experimentally observed for the first time. When the previously observed electronic transition system [a + 12.92]1–X1 (Tsang et al., 2019) is also included, the four observed systems are assigned as the two spin-orbit components X0+ and X1 of the 3Σ− ground state to the two upper states [15.30]1 and [15.10]1. The spin-orbit splitting between the X0+ and X1 states is determined as 2λ = 2181.10(9) cm−1. The spin-orbit splitting in the ground electronic state and the primarily attributed Λ-S states in the excited states are discussed.

Electronic transition and cascading decay of CuO molecule in the wavelength region 430–550 nm investigated by laser-induced fluorescence spectroscopy

The electronic spectrum of the copper monoxide molecule (CuO) was investigated in the range of 430–550 nm using laser induced fluorescence spectroscopy and single vibronic level emission spectra. The CuO molecule was produced by a supersonic jet-cooled discharge assisted laser ablation of Cu target. Twenty two vibronic bands were observed and assigned to the electronic transition progressions from the ground state X2Пi to excited states C2П3/2, C2П1/2, D2Δ5/2, E2Δ5/2, F2П3/2, G2Σ−, H2П3/2, and I2П3/2, of which ten bands were recorded firstly. The molecular constants of the excited electronic states for 63CuO and 65CuO isotopologues were obtained by a rotational analysis of the spectra. Cascading processes in the electronic excited states were found combining the time-resolved single vibronic level emission spectroscopy. The time-dependent radiative decay curves corresponding to different decay processes were recorded and analyzed.

The high-resolution LIF spectrum of the SiCCl free radical: Probing the silicon-carbon triple bond

A rotationally resolved spectrum of a band of the A∼2Σ+ − X∼2Π electronic transition of the SiCCl free radical has been obtained for the first time. The radical was produced in an electric discharge jet using 1,1-dichlorosilacyclobutane vapor in high pressure argon as the precursor. The laser-induced fluorescence spectrum of the band system in the 610–550 nm region was recorded and the 2Π3/2 spin component of the 0–0 band was studied at high resolution. Rotational analysis gave the B values for the combining states and by fixing the CCl bond lengths at ab initio values we obtained r″ (SiC) = 1.692(1) Å and r' (SiC) = 1.594(1) Å. The bond lengths correspond to a silicon-carbon double bond in the ground state and an unusual SiC triple bond in the excited state. Single vibronic level emission spectra yielded the ground state bending and stretching energy levels up to energies near 5000 cm−1. These were satisfactorily fitted to a Renner-Teller model that included spin–orbit and vibrational anharmonicity effects.

The laser-induced fluorescence spectroscopy of gold monoxide: B2Σ−－X2Π3/2 transition

The electronic spectrum of the gaseous gold monoxide molecule has been investigated in the range of 16000－18500 cm−1 using laser induced fluorescence spectroscopy and single vibronic level emission spectra. Five rotationally resolved vibronic bands are observed and assigned to the transitions B2Σ− (v' = 0－4) －X2п3/2 (v'' = 0), in which the 0－0 transition is observed for the first time. The molecular constants of the excited state B2Σ− are obtained by a rotational analysis of the spectra. The spin-orbit coupling constant and the vibrational constants of the ground state X2пi are determined with the accuracy improved by one order of magnitude. The lifetimes of most observed bands are also measured for the first time.

Laser-induced fluorescence detection of the elusive SiCF free radical.

The SiCF free radical has been spectroscopically identified for the first time. The radical was produced in an electric discharge jet using CF3Si(CH3)3 or CF3SiH3 vapor in high pressure argon as the precursor. The laser-induced fluorescence spectrum of the Ã∑+2-X̃∏2 band system in the 610 - 550 nm region was recorded and the ∏3/22 spin component of the 0-0 band was studied at high resolution. Rotational analysis gave the B values for the combining states, and by fixing the CF bond lengths at ab initio values we obtained r″Si-C=1.6921Å and r'Si-C=1.594(1)Å. The bond lengths correspond to a silicon-carbon double bond in the ground state and an unusual Si-C triple bond in the excited state. Single vibronic level emission spectra yielded the ground state bending and stretching energy levels. These were fitted to a Renner-Teller model that included spin-orbit and limited vibrational anharmonicity effects.

An experimental and theoretical study of the Ã(2)A(″)Π-X̃(2)A(') band system of the jet-cooled HBBr/DBBr free radical.

The electronic spectra of the HBBr and DBBr free radicals have been studied in depth. These species were prepared in a pulsed electric discharge jet using a precursor mixture of BBr3 vapor and H2 or D2 in high pressure argon. Transitions to the electronic excited state of the jet-cooled radicals were probed with laser-induced fluorescence and the ground state energy levels were measured from the single vibronic level emission spectra. HBBr has an extensive band system in the red which involves a linear-bent transition between the two Renner-Teller components of what would be a (2)Π state at linearity. We have used high level ab initio theory to calculate potential energy surfaces for the bent (2)A' ground state and the linear Ã(2)A(″)Π excited state and we have determined the ro-vibronic energy levels variationally, including spin orbit effects. The correspondence between the computed and experimentally observed transition frequencies, upper state level symmetries, and H and B isotope shifts was used to make reliable assignments. We have shown that the ground state barriers to linearity, which range from 10 000 cm(-1) in HBF to 2700 cm(-1) in BH2, are inversely related to the energy of the first excited (2)Σ ((2)A') electronic state. This suggests that a vibronic coupling mechanism is responsible for the nonlinear equilibrium geometries of the ground states of the HBX free radicals.

Simulation of the single-vibronic-level emission spectra of HAsO and DAsO.

The single-vibronic-level (SVL) emission spectra of HAsO and DAsO have been simulated by electronic structure/Franck-Condon factor calculations to confirm the spectral molecular carrier and to investigate the electronic states involved. Various multi-reference (MR) methods, namely, NEVPT2 (n-electron valence state second order perturbation theory), RSPT2-F12 (explicitly correlated Rayleigh-Schrodinger second order perturbation theory), and MRCI-F12 (explicitly correlated multi-reference configuration interaction) were employed to compute the geometries and relative electronic energies for the X̃(1)A(') and Ã(1)A(″) states of HAsO. These are the highest level calculations on these states yet reported. The MRCI-F12 method gives computed T0 (adiabatic transition energy including zero-point energy correction) values, which agree well with the available experimental T0 value much better than previously computed values and values computed with other MR methods in this work. In addition, the potential energy surfaces of the X̃(1)A(') and Ã(1)A(″) states of HAsO were computed using the MRCI-F12 method. Franck-Condon factors between the two states, which include anharmonicity and Duschinsky rotation, were then computed and used to simulate the recently reported SVL emission spectra of HAsO and DAsO [R. Grimminger and D. J. Clouthier, J. Chem. Phys. 135, 184308 (2011)]. Our simulated SVL emission spectra confirm the assignments of the molecular carrier, the electronic states involved, and the vibrational structures observed in the SVL emission spectra but suggest a loss of intensity in the reported experimental spectra at the low emission energy region almost certainly due to a loss of responsivity near the cutoff region (∼800 nm) of the detector used. Computed and experimentally derived re (equilibrium) and/or r0 {the (0,0,0) vibrational level} geometries of the two states of HAsO are discussed.

Towards a global model of spin-orbit coupling in the halocarbenes.

We report a global analysis of spin-orbit coupling in the mono-halocarbenes, CH(D)X, where X = Cl, Br, and I. These are model systems for examining carbene singlet-triplet energy gaps and spin-orbit coupling. Over the past decade, rich data sets collected using single vibronic level emission spectroscopy and stimulated emission pumping spectroscopy have yielded much information on the ground vibrational level structure and clearly demonstrated the presence of perturbations involving the low-lying triplet state. To model these interactions globally, we compare two approaches. First, we employ a diabatic treatment of the spin-orbit coupling, where the coupling matrix elements are written in terms of a purely electronic spin-orbit matrix element which is independent of nuclear coordinates, and an integral representing the overlap of the singlet and triplet vibrational wavefunctions. In this way, the structures, harmonic frequencies, and normal mode displacements from ab initio calculations were used to calculate the vibrational overlaps of the singlet and triplet state levels, including the full effects of Duschinsky mixing. These calculations have allowed many new assignments to be made, particularly for CHI, and provided spin-orbit coupling parameters and values for the singlet-triplet gaps. In a second approach, we have computed and fit full geometry dependent spin-orbit coupling surfaces and used them to compute matrix elements without the product form approximation. Those matrix elements were used in similar fits varying the anharmonic constants and singlet-triplet gap to reproduce the experimental levels. The derived spin-orbit parameters for carbenes CHX (X = Cl, Br, and I) show an excellent linear correlation with the atomic spin-orbit constant of the corresponding halogen, indicating that the spin-orbit coupling in the carbenes is consistently around 14% of the atomic value.

Applied quantum chemistry: Spectroscopic detection and characterization of the F2BS and Cl2BS free radicals in the gas phase.

In this and previous work [D. J. Clouthier, J. Chem. Phys. 141, 244309 (2014)], the spectroscopic signatures of the X2BY (X = H, halogen, Y = O, S) free radicals have been predicted using high level ab initio theory. The theoretical results have been used to calculate the electronic absorption and single vibronic level (SVL) emission spectra of the radicals under typical jet-cooled conditions. Using these diagnostic predictions, the previously unknown F2BS and Cl2BS free radicals have been identified and characterized. The radicals were prepared in a free jet expansion by subjecting precursor mixtures of BF3 or BCl3 and CS2 vapor to an electric discharge at the exit of a pulsed molecular beam valve. The B̃(2)A1-X̃(2)B2 laser-induced fluorescence spectra were found within 150 cm(-1) of their theoretically predicted positions with vibronic structure consistent with our Franck-Condon simulations. The B̃(2)A1 state emits down to the ground state and to the low-lying Ã(2)B1 excited state and the correspondence between the observed and theoretically derived SVL emission Franck-Condon profiles was used to positively identify the radicals and make assignments. Excited state Coriolis coupling effects complicate the emission spectra of both radicals. In addition, a forbidden component of the electronically allowed B̃-X̃ band system of Cl2BS is evident, as signaled by the activity in the b2 modes in the spectrum. Symmetry arguments indicate that this component gains intensity due to a vibronic interaction of the B̃(2)A1 state with a nearby electronic state of (2)B2 symmetry.

An experimental and theoretical study of the electronic spectrum of the HBCl free radical.

Following our previous discovery of the spectra of the HBX (X = F, Cl, and Br) free radicals [S.-G. He, F. X. Sunahori, and D. J. Clouthier, J. Am. Chem. Soc. 127, 10814 (2005)], the Ã(2)A(″)Π-X̃(2)A(') band systems of the HBCl and DBCl free radicals have been studied in detail. The radicals have been prepared in a pulsed electric discharge jet using a precursor mixture of BCl3 and H2 or D2 in high pressure argon. Laser-induced fluorescence (LIF) and single vibronic level emission spectra have been recorded to map out the ground and excited state vibrational energy levels. The band system involves a linear-bent transition between the two Renner-Teller components of what would be a (2)Π electronic state at linearity. We have used high level ab initio theory to calculate the ground and excited state potential energy surfaces and have determined the vibronic energy levels variationally. The theory results were used to assign the LIF spectra which involve transitions from the ground state zero-point level to high vibrational levels of the excited state. The correspondence between theory and experiment, including the transition frequencies, upper state band symmetries, and H, B, and Cl isotope shifts, was used to validate the assignments.

In search of the X2BO and X2BS (X = H, F) free radicals: ab initio studies of their spectroscopic signatures.

The F2BO free radical is a known, although little studied, species but similar X2BY (X = H, D, F; Y = O, S) molecules are largely unknown. High level ab initio methods have been used to predict the molecular structures, vibrational frequencies (in cm(-1)), and relative energies of the ground and first two excited electronic states of these free radicals, as an aid to their eventual spectroscopic identification. The chosen theoretical methods and basis sets were tested on F2BO and found to give good agreement with the known experimental quantities. In particular, complete basis set extrapolations of coupled-cluster single and doubles with perturbative triple excitations/aug-cc-pVXZ (X = 3, 4, 5) energies gave excellent electronic term values, due to small changes in geometry between states and the lack of significant multireference character in the wavefunctions. The radicals are found to have planar C2v geometries in the X̃(2)B2 ground state, the low-lying Ã(2)B1 first excited state, and the higher B̃(2)A1 state. Some of these radicals have very small ground state dipole moments hindering microwave measurements. Infrared studies in matrices or in the gas phase may be possible although the fundamentals of H2BO and H2BS are quite weak. The most promising method of identifying these species in the gas phase appears to be absorption or laser-induced fluorescence spectroscopy through the allowed B̃-X̃ transitions which occur in the visible-near UV region of the electromagnetic spectrum. The ab initio results have been used to calculate the Franck-Condon profiles of the absorption and emission spectra, and the rotational structure of the B̃-X̃0(0)(0) bands has been simulated. The calculated single vibronic level emission spectra provide a unique, readily recognizable fingerprint of each particular radical, facilitating the experimental identification of new X2BY species in the gas phase.

Revisiting the Renner–Teller effect in the [formula omitted] state of CCN: Pulsed discharge-supersonic jet single vibronic level emission spectroscopy

The CCN radical is a prototypical system for study of the Renner–Teller effect in its degenerate X∼2Π ground state, with a number of experimental and theoretical studies carried out over the past 50years. Most experimental studies have focused on the low-lying vibrational structure as observed in the high-resolution spectra of hot bands, or in emission. In this work, we have used pulsed-discharge supersonic-jet single vibronic level (SVL) emission spectroscopy from selected levels of the A∼2Δ state to probe the vibronic structure of X∼2Π up to ∼6000cm−1 above the vibrationless level. Around 50 levels were assigned, and these were fit to a Renner–Teller Hamiltonian to derive a detailed set of spectroscopic constants. Our data are compared with the results of recent high level coupled cluster calculations.

Simulation of the single-vibronic-level emission spectrum of HPS.

We have computed the potential energy surfaces of the X¹A' and ùA" states of HPS using the explicitly correlated multi-reference configuration interaction (MRCI-F12) method, and Franck-Condon factors between the two states, which include anharmonicity and Duschinsky rotation, with the aim of testing the assignment of the recently reported single-vibronic-level (SVL) emission spectrum of HPS [R. Grimminger, D. J. Clouthier, R. Tarroni, Z. Wang, and T. J. Sears, J. Chem. Phys. 139, 174306 (2013)]. These are the highest level calculations on these states yet reported. It is concluded that our spectral simulation supports the assignments of the molecular carrier, the electronic states involved and the vibrational structure of the experimental laser induced fluorescence, and SVL emission spectra proposed by Grimminger et al. [J. Chem. Phys. 139, 174306 (2013)]. However, there remain questions unanswered regarding the relative electronic energies of the two states and the geometry of the excited state of HPS.

An experimental and ab initio study of the electronic spectrum of the jet-cooled F2BO free radical.

We have studied the B̃ (2)A1-X̃ (2)B2 laser-induced fluorescence (LIF) spectrum of the jet-cooled F2BO radical for the first time. The transition consists of a strong 0(0)(0)band at 446.5 nm and eight weak sequence bands to shorter wavelengths. Single vibronic level emission spectra obtained by laser excitation of individual levels of the B̃ state exhibit two electronic transitions: a very weak, sparse B̃-X̃ band system in the 450-500 nm region and a stronger, more extensive set of B̃ (2)A1-à (2)B1 bands in the 580-650 nm region. We have also performed a series of high level ab initio calculations to predict the electronic energies, molecular structures, vibrational frequencies, and rotational and spin-rotation constants in the X̃ (2)B2, à (2)B1 and B̃ (2)A1 electronic states as an aid to the analysis of the experimental data. The theoretical results have been used as input for simulations of the rotationally resolved B̃ (2)A1-X̃ (2)B2 0(0)(0) LIF band and Franck-Condon profiles of the LIF and single vibronic level emission spectra. The agreement between the simulations obtained with purely ab initio parameters and the experimental spectra validates the geometries calculated for the ground and excited states and the conclusion that the radical has C2v symmetry in the X̃, Ã, and B̃ states. The spectra provide considerable new information about the vibrational energy levels of the X̃ and à states, but very little for the B̃ state, due to the very restrictive Franck-Condon factors in the LIF spectra.