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GaCH2, a free radical thought to play a role in the chemical vapor deposition of gallium-containing thin films and semiconductors, has been spectroscopically detected for the first time. The radical was produced in a pulsed discharge jet using a precursor mixture of trimethylgallium vapor in high pressure argon and studied by laser-induced fluorescence and wavelength resolved emission techniques. Partially rotationally resolved spectra of the hydrogenated and deuterated species were obtained, and they exhibit the nuclear statistical weight variations and subband structure expected for a 2A2–2B1 electronic transition. The measured spectroscopic quantities have been compared to our own ab initio calculations of the ground and excited state properties. The electronic spectrum of gallium methylene is similar to the corresponding spectrum of the aluminum methylene radical, which we reported in 2022. [ABSTRACT FROM AUTHOR]

The H2CSn and D2CSn molecules have been detected for the first time by laser-induced fluorescence (LIF) and emission spectroscopic techniques through the B ̃ 1 B 2 − X ̃ 1 A 1 electronic transition in the 425–400 nm 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 A ̃ 1 A 2 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). [ABSTRACT FROM AUTHOR]

The AlCH2 free radical has been spectroscopically identified for the first time. This highly reactive species was produced in an electric discharge jet using trimethylaluminum vapor in high pressure argon as the precursor. The laser-induced fluorescence spectrum of the B ̃ 2 A 2 − X ̃ 2 B 1 band system in the 513–483 nm region was recorded, and the 0–0 bands of AlCH2 and AlCD2 were studied at high resolution. The fine structure splittings were found to be due primarily to the Fermi contact interaction in the excited state rather than the usual spin–rotation coupling. Rotational analysis gave the molecular constants of the combining states, and the geometries were obtained as r ″ A 1 − C = 1.959 1 A ◦ , r ″ C − H = 1.106 1 A ◦ , θ HCH ″ = 110.4 1 ◦ and r ′ A 1 − C = 1.943 1 A ◦ , r ′ C − H = 1.091 1 A ◦ , θ H C H ′ = 115.4 1 ◦ . The bond lengths correspond to an aluminum–carbon single bond in both states. [ABSTRACT FROM AUTHOR]

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 A ̃ 1A″– X ̃ 1A′ spectrum of HSnBr only consists of the 0 0 0 and 2 0 1 cold bands that show clearly resolved subband structure with fluorescence lifetimes varying from 526 to 162 ns. The DSnBr LIF spectrum exhibits four bands ( 0 0 0 , 2 0 1 , 2 0 2 , and 1 0 1 ) whose fluorescence lifetimes decrease from 525 ns (00) to 175 ns (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 317 cm−1 in the excited state emit detectable photons down to the ground state. [ABSTRACT FROM AUTHOR]

The possibilities of emission from the Ã1A″ and ã3A″ excited states of the triatomic halosilylenes, halogermylenes, and halostannylenes (HMX, M = Si, Ge, Sn; X = F, Cl, Br, I) have been explored in a series of extensive ab initio calculations. The triplet states are found to have deep bonding wells supporting an extensive manifold of vibrational levels, which could give rise to observable triplet–singlet phosphorescence. The ã– X ̃ band systems of the halosilylenes are calculated to occur at the red edge of the visible and are likely to be very weak. In contrast, the HGeX and HSnX triplet–singlet spectra are shifted 1000–2000 cm−1 to the higher energy and are expected to be significantly stronger due to increased spin–orbit coupling, making the spectra viable targets for experimental investigations. The Ö X ̃ fluorescence is found to be limited by the possibility of HMX (Ã1A″) → H (2S) + MX (2 Π) dissociation in the excited state, leading to the expectation that HGeF is unlikely to be detectable by laser-induced fluorescence (LIF) spectroscopy. The HSiX and HGeX species with known LIF spectra are found to have deeper à state bonding wells and minimal or no calculated barriers to dissociation. It is generally found that the intensity in their LIF spectra tails off due to a diminution of vibrational overlap rather than the abrupt opening of a dissociation channel. Few of the HSnX species are known experimentally. HSnF and DSnF are found to dissociate very low down in the à state vibrational manifold and are predicted to be unobservable by LIF spectroscopy. The LIF spectrum of HSnCl is expected to consist of only one or two bands, with slightly more activity for DSnCl, precisely as has recently been found experimentally. HSnBr and DSnBr have deeper à state bonding wells, and their LIF spectra are thus likely to be more extensive. Although HSnI and DSnI are calculated to have deep bonding wells with respect to the H + MX dissociation, predictions are complicated by the existence of a global small bond angle minimum and the opening of a second SnH + I dissociation channel. [ABSTRACT FROM AUTHOR]

Extensive ab initio investigations of the ground and electronic excited states of the AlCH2 free radical have been carried out in order to predict the spectroscopic properties of this, as yet, undetected species. Difficulties with erratic predictions of the ground state vibrational frequencies, both in the literature and in the present work, have been traced to serious broken-symmetry instabilities in the unrestricted Hartree-Fock orbitals at the ground state equilibrium geometry. The use of restricted open-shell Hartree–Fock or complete active space self consistent field orbitals avoids these problems and leads to consistent and realistic sets of vibrational frequencies for the ground state. Using the internally contracted multireference configuration interaction method with aug-cc-pV(T+d)Z basis sets, we have calculated the geometries, energies, dipole moments, and vibrational frequencies of eight electronic states of AlCH2 and AlCD2. In addition, we have generated Franck–Condon simulations of the expected vibronic structure of the A ̃ − X ̃ , B ̃ − X ̃ , C ̃ − X ̃ , and C ̃ − A ̃ band systems, which will be useful in searches for the electronic spectra of the radical. We have also simulated the expected rotational structure of the 0–0 absorption bands of these transitions at modest resolution under supersonic expansion cooled conditions. Our conclusion is that if AlCH2 can be generated in sufficient concentrations in the gas phase, it is most likely detectable through the B ̃ 2 A 2 – X ̃ 2 B 1 or C ̃ 2 A 1 – X ̃ 2 B 1 electronic transitions at 515 nm and 372 nm, respectively. Both band systems have vibrational and rotational signatures, even at modest resolution, that are diagnostic of the aluminum methylene free radical. [ABSTRACT FROM AUTHOR]

The à 2A1– X ̃ 2E 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 v 5 ′ and v 6 ′ 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. [ABSTRACT FROM AUTHOR]

The A ̃ 2 A 1 − X ̃ 2 B 1 electronic transition of the jet-cooled stibino (SbH2 and SbD2) free radical has been observed for the first time using laser induced fluorescence (LIF) detection. The radicals were produced by a pulsed electric discharge through a mixture of stibine (SbH3 or SbD3) in high pressure argon at the exit of a pulsed molecular beam valve. SbH2 exhibits only three LIF bands, assigned as 2 1 0 , 0 0 0 , and 2 0 1 , with a fluorescence lifetime (τ), which decreases from ∼50 ns for 00 to <10 ns for 21. LIF transitions to the 00 (τ ∼ 2 µs), 21 (τ ∼ 400 ns), and 22 (τ ∼ 75 ns) upper vibronic states of SbD2 were also observed. High-resolution spectra exhibited large spin-rotation splittings and small resolved antimony hyperfine splittings due to a substantial Fermi contact interaction in the excited state. The experimentally determined rotational constants gave effective molecular structures of r 0 ″ = 1.724(2) Å, θ 0 ″ = 90.38(7)° and r 0 ′ = 1.693(6) Å, θ 0 ′ = 120.6(3)°. The ground state bending vibrational levels up to eight quanta (6404 cm−1) in SbH2 and 12 quanta (6853 cm−1) in SbD2 were measured from dispersed fluorescence spectra. All indications are that SbH2 undergoes a dissociative process at low vibrational energies in the excited electronic state. [ABSTRACT FROM AUTHOR]

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 à ∑ + 2 − X ̃ ∏ 2 band system in the 610 − 550 nm region was recorded and the ∏ 3 / 2 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 CF bond lengths at ab initio values we obtained r ″ Si–C = 1. 692 1 Å 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. [ABSTRACT FROM AUTHOR]

The SnH2 and SnD2 molecules have been detected for the first time in the gas phase by laser-induced fluorescence (LIF) and emission spectroscopic techniques through the ùB1-X 1A1 electronic transition. These reactive species were prepared in a pulsed electric discharge jet using (CH3)4Sn or SnH4/SnD4 precursors diluted in high pressure argon. Transitions to the electronic excited state of the jet-cooled molecules were probed with LIF, and the ground state energy levels were measured from single rovibronic level emission spectra. The LIF spectrum of SnD2 afforded sufficient rotational structure to determine the ground and excited state geometries: r 0" = 1.768 Å, θ "0 = 91.0°, r0' = 1.729 Å, θ0' = 122.9°. All of the observed LIF bands show evidence of a rotational-level-dependent predissociation process which rapidly decreases the fluorescence yield and lifetime with increasing rotational angular momentum in each excited vibronic level. This behavior is analogous to that observed in SiH2 and GeH2 and is suggested to lead to the formation of ground state tin atoms and hydrogen molecules. [ABSTRACT FROM AUTHOR]

The ground state bending levels of 11BH2 have been studied experimentally using a combination of low-resolution emission spectroscopy and high-resolution stimulated emission pumping (SEP) measurements. The data encompass the energy range below, through, and above the calculated position of the barrier to linearity. For the bending levels (0,3,0) and above, the data show substantial K-reordering, with the Ka″ = 1 levels falling well below those with Ka″ = 0. A comparison of the high-resolution rotationally resolved SEP data to our own very high level ab initio calculations of the rovibronic energy levels shows agreement approaching near-spectroscopic accuracy (a few cm-1). The data reported in this work provide very stringent tests for future theoretical treatments of this prototypical seven-electron free radical. [ABSTRACT FROM AUTHOR]

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 ²Π state at linearity. We have used high level ab initio theory to calculate potential energy surfaces for the bent ²A′ ground state and the linear òA"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 ²Σ(²A′) 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. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence and wavelength resolved emission spectra of the B 4Σ−-X 4Σ− band system of the gas phase cold aluminum carbide free radical have been obtained using the pulsed discharge jet technique. The radical was produced by electron bombardment of a precursor mixture of trimethylaluminum in high pressure argon. High resolution spectra show that each rotational line of the 0-0 and 1-1 bands of AlC is split into at least three components, with very similar splittings and intensities in both the P- and R-branches. The observed structure was reproduced by assuming bβS magnetic hyperfine coupling in the excited state, due to a substantial Fermi contact interaction of the unpaired electron in the aluminum 3s orbital. Rotational analysis has yielded ground and excited state equilibrium bond lengths in good agreement with the literature and our own ab initio values. Small discrepancies in the calculated intensities of the hyperfine lines suggest that the upper state spin-spin constant λ' is of the order of ≈0.025-0.030 cm−1. [ABSTRACT FROM AUTHOR]

The cold boron carbide free radical (BC X 4S-) has been produced in a pulsed discharge free jet expansion using a precursor mixture of trimethylborane in high pressure argon. High resolution laser induced fluorescence spectra have been obtained for the B 4Σ--X 4Σ- and E 4π-X 4Σ- band systems of both 11BC and 10BC. An optical-optical double resonance (OODR) scheme was implemented to study the finer details of both band systems. This involved pumping a single rotational level of the B state with one laser and then recording the various allowed transitions from the intermediate B state to the final E state with a second laser by monitoring the subsequent E-X ultraviolet fluorescence. In this fashion, we were able to prove unambiguously that, contrary to previous studies, the spin-spin constant λ is negative in the ground state and positive in the B 4Σ- excited state. It has been shown that λ" < 0 is in fact expected based on a semiempirical second order perturbation theory calculation of the magnitude of the spin-spin constant. The OODR spectra have also been used to validate our assignments of the complex and badly overlapped E 4π-X 4Σ- 0-0 and 1-0 bands of 11BC. The E-X 0-0 band of 10BC was found to be severely perturbed. The ground state main electron configuration is . . .3σ²4σ²5σ¹1π²2π0 and the derived bond lengths show that there is a 0.03 Å contraction in the B state, due to the promotion of an electron from the 4σ antibonding orbital to the 5σ bonding orbital. In contrast, the bond length elongates by 0.15 Å in the E state, a result of promoting an electron from the 5σ bonding orbital to the 2π antibonding orbitals. [ABSTRACT FROM AUTHOR]

The rotationally resolved triplet-singlet electronic transition of SiF2 is experimentally determined using pyrolysis jet technique. Intensity distribution among the rotational branches in the 0 (sub 0)(super 0) band shows that the intensity of triplet-singlet transition is mainly due to the spin-orbit coupling with excited 1B2 states. Rotational analysis in the 0 (sub 0)(super 0) band shows that the upper state has 3B1 symmetry.

The spectroscopy of gas phase BH2 has not been explored experimentally since the pioneering study of Herzberg and Johns in 1967. In the present work, laser-induced fluorescence (LIF) spectra of the òB1(Πu) - X... ²A1 band system of 11BH2, 10BH2, 11BD2, and 10BD2 have been observed for the first time. The free radicals were "synthesized" by an electric discharge through a precursor mixture of 0.5% diborane (B2H6 or B2D6) in high pressure argon at the exit of a pulsed valve. A total of 67 LIF bands have been measured and rotationally analyzed, 62 of them previously unobserved. These include transitions to a wide variety of excited state bending levels, to several stretch-bend combination levels, and to three ground state levels which gain intensity through Renner-Teller coupling to nearby excited state levels. As an aid to vibronic assignment of the spectra, very high level hybrid ab initio potential energy surfaces were built starting from the coupled cluster singles and doubles with perturbative triples (CCSD(T))/aug-cc-pV5Z level of theory for this seven-electron system. In an effort to obtain the highest possible accuracy, the potentials were corrected for core correlation, extrapolation to the complete basis set limit, electron correlation beyond CCSD(T), and diagonal Born-Oppenheimer effects. The spin-rovibronic states of the various isotopologues of BH2 were calculated for energies up to 22 000 cm-1 above the X... (000) level without any empirical adjustment of the potentials or fitting to experimental data. The agreement with the new LIF data is excellent, approaching near-spectroscopic accuracy (a few cm-1) and has allowed us to understand the complicated spin-rovibronic energy level structure even in the region of strong Renner-Teller resonances. [ABSTRACT FROM AUTHOR]

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 B²A1-X ²B2 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 B² A1 state emits down to the ground state and to the low-lying òB1 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 B-X 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 B²A1 state with a nearby electronic state of ²B2 symmetry. [ABSTRACT FROM AUTHOR]

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 A1 A''n - X2A' 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 2n 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. [ABSTRACT FROM AUTHOR]

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 X2B2 ground state, the low-lying AA²B1 first excited state, and the higher B²A1 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 B-X 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 B-X000 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. [ABSTRACT FROM AUTHOR]

We have studied the ... 2A1–... 2B2 laser-induced fluorescence (LIF) spectrum of the jet-cooled F2BO radical for the first time. The transition consists of a strong 000 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 ... state exhibit two electronic transitions: a very weak, sparse ... band system in the 450–500 nm region and a stronger, more extensive set of ... 2A1– ... 2B1 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 ... 2B2, Ã2B1 and ... 2A1 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 ... 2A1–... 2B2 000 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 ..., Ã, and ... states. The spectra provide considerable new information about the vibrational energy levels of the ... and à states, but very little for the ... state, due to the very restrictive Franck-Condon factors in the LIF spectra. [ABSTRACT FROM AUTHOR]

Spectra of the Ã2A1-X2B1 transition of the jet-cooled AsD2 and AsHD isotopologues of the arsino radical have been studied by laser induced fluorescence and wavelength resolved emission techniques. A high-resolution spectrum of the AsD2 000 band has been recorded, and an improved r0 structure [r0′ = 1.487(4) Å, θ0′ = 123.0(2)°] for the à state has been determined from the rotational constants. To aid in the analysis of the vibrational levels, an ab initio potential energy surface of the X2B1 state has been constructed and the rovibronic energy levels of states on that potential have been determined using a variational method. The vibrational levels observed in wavelength resolved emission spectra have been fitted to a local mode Hamiltonian with most anharmonic parameters fixed at ab initio values, and the resulting harmonic frequencies have been used to perform a normal coordinate analysis which yielded an improved set of quadratic force constants and an estimate of the equilibrium ground state structure. [ABSTRACT FROM AUTHOR]

The laser-induced fluorescence spectrum of jet-cooled chlorine cation has been recorded in the 500-312 nm region with high sensitivity and rigorous vibrational and spin-orbit cooling. More than 80 bands of the highly vibrationally perturbed A 2Πu-X 2Πg electronic transition have been detected and shown to originate from the Ω = 3/2 spin-orbit component of v = 0 of the ground state. The spectrum extends some 3700 cm-1 to the red of any previously published report and the 0-0 band has been identified for the first time. The bands have regular rotational structure but exhibit irregular vibrational intervals and isotope splittings. Our ab initio studies show that the perturbations are due to a ΔΩ = 0 spin-orbit interaction between the A2Π3/2u and B2Δ 3/2u states which have an avoided crossing at ∼2.5 Å, which corresponds to v ≈ 4 of the A state. The nonadiabatic coupled equations have been solved for this two-state interaction after constructing the diabatic potentials including only the diagonal (ΔΛ = 0) spin-orbit coupling, yielding low-lying vibrational energy levels, isotope splittings, and rotational constants in good agreement with experiment. The calculations show that many of the observed bands are actually transitions to predominantly B state vibrational levels, which borrow oscillator strength from the A-X transition through spin-orbit mixing. Starting from the coupled equations solutions, we have fitted the experimental data using an effective Hamiltonian matrix that includes the vibrational energy levels of the A and B states and a single electronic spin-orbit coupling term HSOAB which has a value of 240 cm-1. Transitions up to v′ = 32 in both states have been satisfactorily fitted for all three chlorine isotopologues, providing a quantitative description of the perturbations. Transitions to higher levels are complicated by interactions with other electronic states. [ABSTRACT FROM AUTHOR]

The laser-induced fluorescence spectrum of jet-cooled chlorine cation has been recorded in the 500-312 nm region with high sensitivity and rigorous vibrational and spin-orbit cooling. More than 80 bands of the highly vibrationally perturbed A 2Πu-X 2Πg electronic transition have been detected and shown to originate from the Ω = 3/2 spin-orbit component of v = 0 of the ground state. The spectrum extends some 3700 cm-1 to the red of any previously published report and the 0-0 band has been identified for the first time. The bands have regular rotational structure but exhibit irregular vibrational intervals and isotope splittings. Our ab initio studies show that the perturbations are due to a ΔΩ = 0 spin-orbit interaction between the A2Π3/2u and B2Δ 3/2u states which have an avoided crossing at ∼2.5 Å, which corresponds to v ≈ 4 of the A state. The nonadiabatic coupled equations have been solved for this two-state interaction after constructing the diabatic potentials including only the diagonal (ΔΛ = 0) spin-orbit coupling, yielding low-lying vibrational energy levels, isotope splittings, and rotational constants in good agreement with experiment. The calculations show that many of the observed bands are actually transitions to predominantly B state vibrational levels, which borrow oscillator strength from the A-X transition through spin-orbit mixing. Starting from the coupled equations solutions, we have fitted the experimental data using an effective Hamiltonian matrix that includes the vibrational energy levels of the A and B states and a single electronic spin-orbit coupling term HSOAB which has a value of 240 cm-1. Transitions up to v′ = 32 in both states have been satisfactorily fitted for all three chlorine isotopologues, providing a quantitative description of the perturbations. Transitions to higher levels are complicated by interactions with other electronic states. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence and wavelength resolved emission spectra of the à 2Σ+ - X 2Πi electronic transition of the jet-cooled nitrous oxide cation have been recorded. The ions were produced in a pulsed electric discharge at the exit of a supersonic expansion using a precursor mixture of N2O in high pressure argon. Both spin-orbit components of the 000 band were studied at high resolution and rotationally analyzed to provide precise molecular constants for the combining states. Emission spectra were obtained by laser excitation of the 000, 201, 301, and 202 absorption bands, providing extensive data on the ground state bending, stretching, and combination vibrational levels. These data were fitted to a Renner-Teller model including spin-orbit, anharmonic, and Fermi resonance terms. The observed energy levels and fitted parameters were found to be comparable to those in the literature predicted from an ab initio potential energy surface. [ABSTRACT FROM AUTHOR]

The previously unknown electronic spectrum of the H2PO free radical has been identified in the 407-337 nm region using a combination of laser-induced fluorescence and single vibronic level emission spectroscopy. High level ab initio predictions of the properties of the ground and first two excited doublet states were used to identify the spectral region in which to search for the electronic transition and were used to aid in the analysis of the data. The band system is assigned as the B2A′-X2A′ electronic transition which involves promotion of an electron from the π to the π* molecular orbital. The excited state r0 molecular structure was determined by rotational analysis of high resolution LIF spectra to be r(PO) = 1.6710(2) Å, r(PH) = 1.4280(6) Å, θ(HPO) = 105.68(7)°, θ(HPH) = 93.3(2)°, and the out-of-plane angle = 66.8(2)°. The structural changes on electronic excitation, which include substantial increases in the PO bond length and out-of-plane angle, are as expected based on molecular orbital theory and our previous studies of the isoelectronic H2AsO, Cl2PS, and F2PS free radicals. [ABSTRACT FROM AUTHOR]

The B2A′-X2A′ transition of the prototypical thiophosphoryl radical, H2PS, was observed for the first time using laser-induced fluorescence and single vibronic level emission spectroscopy. H2PS and its deuterated isotopologues, D2PS and HDPS, were produced in a pulsed supersonic discharge jet from a precursor mixture of Cl3PS and H2 or D2 or an H2/D2 mixture in high-pressure argon. High level ab initio calculations of the lowest three doublet electronic states helped in the definitive assignment of the B-X transition, which involves electron promotion from the π to the π* orbital. Vibrational frequencies were determined for several modes of each isotopologue in the X and B states and found to be in accord with theoretical predictions. Although a line-by-line rotational analysis was not possible, the observed band contours are consistent with the geometries obtained from our ab initio calculations. Theory indicates that PS bond length increases upon electronic excitation, while the pyramidalization of the radical does not change significantly. [ABSTRACT FROM AUTHOR]

The à 1A′′-X 1A′ electronic spectrum of the jet-cooled transient molecule HAsO and its deuterated isotopologue has been observed for the first time by pulsed discharge jet laser spectroscopy. The techniques of laser-induced fluorescence and single vibronic level emission were employed to probe the electronic properties of the species. The bending and AsO stretching frequencies have been determined in both states. A rotational analysis of the 000 bands of both HAsO and DAsO has been completed and the following effective (r0) structures were derived: r″(HAs) = 1.576(3) Å, r″(AsO) = 1.8342(5) Å, and θ″ = 101.5(4)°; and r′(HAs) = 1.569(4) Å, r′(AsO) = 1.7509(9) Å, and θ′ = 93.1(10)°. In the rotational analysis, lines induced by axis-tilting were observed, and calculated spectra with an axis tilting angle of 3.0(5)° reproduced the intensity of these lines. The change in geometry on electronic excitation is similar to that observed for the molecule HPO, with an increase in the X-O bond length and a decrease in the HXO angle, but contrary to the predictions of the Walsh diagram for generic HAB triatomic molecules. Our ab initio calculations show that the correlation between orbital energy and bond angle changes upon electronic excitation, resulting in the atypical angle change. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence and wavelength resolved emission spectra of the C 2B2-X 2A1 band system of the gas phase aluminum dicarbide free radical have been obtained using the pulsed discharge jet technique. The radical was produced by electron bombardment of a precursor mixture of trimethylaluminum in high-pressure argon. The three vibrational frequencies of T-shaped AlC2 have been determined in both the combining states along with several of the anharmonicity constants. The 000 band has been recorded with high resolution and rotationally analyzed. The spectrum is complicated by partially resolved spin-rotation and aluminum hyperfine splittings. Where necessary, we have fixed the spin-rotation constants used in the rotational analysis at the values predicted by density functional theory. The derived molecular structures are: r0′′(C-C) = 1.271(2) Å, r0′′(Al-C) = 1.926(1) Å, θ″(C-Al-C) = 38.5(2)°, r0′(C-C) = 1.323(2) Å, r0′(Al-C) = 1.934(1) Å, and θ′(C-Al-C) = 40.0(2)°. Unlike SiC2, aluminum dicarbide shows no spectroscopic evidence of facile isomerization to the linear structure in the ground electronic state. [ABSTRACT FROM AUTHOR]

The near-ultraviolet band system of the jet-cooled boron difluoride free radical has been studied by a combination of laser-induced fluorescence and single vibronic level wavelength resolved emission spectroscopies. The radical was produced in a supersonic discharge jet using a precursor mixture of 1%-3% of BF3 or 10BF3 in high pressure argon. A large number of bands were found in the 340-286 nm region and assigned as transitions from the X2A1 ground state to the lower Renner-Teller component of the Ã2Π excited state, based on our previous ab initio potential energy surface predictions, matching the emission spectra Franck-Condon profiles of 11BF2 and 10BF2, and comparison of observed and calculated boron isotope effects. Several bands have been rotationally analyzed providing ground state structural parameters of r0′′ (BF) = 1.3102(9) Å and θ0′′ (FBF) = 119.7(6)°. The ground state totally symmetric vibrational energy levels of both boron isotopologues have also been measured and assigned up to energies of more than 8000 cm-1. Although BF2 might be considered to be a 'simple' free radical, understanding the details of its electronic spectrum remains a major challenge for both theory and experiment. [ABSTRACT FROM AUTHOR]

The previously unknown arsenic carbide (AsC) free radical has been identified in the gas phase through a combination of laser-induced fluorescence (LIF), single vibronic level emission, and stimulated emission pumping (SEP) spectroscopy in a supersonic expansion. The As12C and As13C isotopologues have been detected as products of an electric discharge in mixtures of arsine (AsH3) and carbon dioxide (12CO2 or 13CO2) in high pressure argon. The B 2Σ+-X 2Σ+ band system was recorded by LIF spectroscopy and emission transitions from the B state down to the ground state and to the low-lying A 2Πi state were observed. High resolution studies of the B-X 0-0 band by LIF and the B-A 0-0 band by SEP spectroscopy enabled a determination of the molecular structures in the three states. Although CN, CP, and AsC have similar X 2Σ+ and A 2Πi. states, the B 2Σ+ state molecular orbital configuration of CP and AsC differs from that of the CN free radical. [ABSTRACT FROM AUTHOR]

The potential energy surfaces (PESs) of the ground X 2A1 and the first excited A 2Π (1 2B1,2 2A1) electronic states of the BF2 radical have been studied ab initio, using a large basis set and CCSD(T) and EOM-CCSD techniques. The calculated PESs were used to variationally calculate the energy levels up to ≈36 000 cm-1 above the ground state. The Renner–Teller splitting parameter ([variant_greek_epsilon]=0.928) found for the A 2Π state of this radical is very large which results in an unusual excited state energy level structure. [ABSTRACT FROM AUTHOR]

The A 2Πu-X 2Πg band systems of jet-cooled 12CO2+ and 13CO2+ have been recorded by laser-induced fluorescence (LIF) techniques. Very intense, vibrationally cold expansions of these cations have been obtained using a pulsed electric discharge jet with a precursor mixture of carbon dioxide or 13C labeled CO2 in high pressure argon. The LIF bands have been partially rotationally analyzed to obtain band origins which yielded an accurate measure of the excited state vibronic energy levels. The energy levels of both isotopologues were fitted with a Renner–Teller model that included spin-orbit coupling, Fermi resonance and anharmonic terms. Single vibronic level emission spectra were also recorded for the 13CO2+ ion and the ground state energy levels fitted using the same Renner–Teller model. The isotope relations have been used to test the validity of the derived parameters. The results give a through description of the vibronic energy levels in the ground and first excited electronic states of the carbon dioxide cation. [ABSTRACT FROM AUTHOR]

The A 2Πu-X 2Πg band systems of jet-cooled 12CO2+ and 13CO2+ have been recorded by laser-induced fluorescence (LIF) techniques. Very intense, vibrationally cold expansions of these cations have been obtained using a pulsed electric discharge jet with a precursor mixture of carbon dioxide or 13C labeled CO2 in high pressure argon. The LIF bands have been partially rotationally analyzed to obtain band origins which yielded an accurate measure of the excited state vibronic energy levels. The energy levels of both isotopologues were fitted with a Renner–Teller model that included spin-orbit coupling, Fermi resonance and anharmonic terms. Single vibronic level emission spectra were also recorded for the 13CO2+ ion and the ground state energy levels fitted using the same Renner–Teller model. The isotope relations have been used to test the validity of the derived parameters. The results give a through description of the vibronic energy levels in the ground and first excited electronic states of the carbon dioxide cation. [ABSTRACT FROM AUTHOR]

The difluorothiophosphoryl (F2PS) free radical has been produced in a supersonic discharge jet from a precursor mixture of F3PS and high pressure argon and detected by laser-induced fluorescence and single vibronic level emission spectroscopy. With the aid of high level ab initio predictions of the properties of the ground and first two excited doublet states, the observed band system has been positively identified as B2A′-X2A′. The electronic transition involves promotion of an electron from the π to the π* orbital with concomitant increases in the out-of-plane angle and PS bond length on excitation. The observed vibrational structure, Franck–Condon profile, rotational band contours, T0 energy, and ground and excited state vibrational frequencies are all in accord with expectations based on our theoretical predictions. [ABSTRACT FROM AUTHOR]

A series of ab initio calculations have been undertaken to predict the spectroscopic properties of the ground and first two excited states of the recently discovered arsenyl (H2AsO) free radical. This 13 valence electron species can be viewed as similar to the formaldehyde radical anion with a ground state electron configuration of ...(π)2(n)2(π*)1. The arsenyl radical is nonplanar (pyramidal) in the ground state with a 59° out-of-plane angle and a 1.67 Å AsO bond length. It has a low-lying n-π*(Ã 2A″) excited state (Te∼5000 cm-1) which has a much larger out-of-plane angle (86°) and longer AsO bond length (1.81 Å). The π-π*(B 2A′) excited state at ∼20 500 cm-1 is less pyramidal (out-of-plane angle=70°) and has a somewhat shorter AsO bond (1.77 Å). Similar trends are found for the H2PO and H2NO free radicals, although the latter has a planar ground state, due to sp2 hybridization of the N atom, and a very long B state AsO bond length. The geometric variations of the ground and excited states of the H2EO (E=N, P, As) radicals, as well as the ground states of the corresponding anions and cations, can be readily rationalized from the Walsh diagram of the anion. The variations in the E-O bond length are a result of changes in both the orbital occupancy and pyramidalization of the molecule. The results of the present work have been employed in the analysis of the B 2A′-X 2A′ electronic band system of the H2AsO free radical as reported in the companion paper. [ABSTRACT FROM AUTHOR]

The previously unknown arsenyl (H2AsO) free radical has been identified in the gas phase through a combination of laser-induced fluorescence and single vibronic level emission spectroscopy in a supersonic expansion. Three isotopologues, H2AsO, HDAsO, and D2AsO have been detected as products of an electric discharge in mixtures of arsine or deuterated arsines, CO2, and argon. The observed spectra are assigned as due to the B 2A′-X 2A′ electronic transition in which an electron in the ground state π orbital is promoted to the π* orbital. Rotational analysis of high-resolution spectra proves that the radical is nonplanar in both electronic states with the following r0 structures: r″(As–H)=1.513(4) Å, r″(As–O)=1.672(1) Å, θ″(HAsH)=101.8(4)°, ground state out-of-plane angle=63.1°; r′(As–H)=1.525(10) Å, r′(As–O)=1.806(3) Å, θ′(HAsH)=93.4(10)°, and excited state out-of-plane angle=70.7°. Small hyperfine splittings in the spectra have enabled the determination of the arsenic Fermi contact parameter in both states. The results of our ab initio studies of the ground and excited state of this radical (see immediately preceding paper) are in good agreement with the spectroscopic analysis. [ABSTRACT FROM AUTHOR]

The fluoroborane (HBF) free radical has a large vibronic interaction which splits the orbitally degenerate 2Π state in the linear configuration into two separate electronic states, one strongly bent and one linear. The observed vibrational structure of the electronic transition between the Renner–Teller pair of states is very complex. As an aid to understanding the spectrum, the vibronic energy levels of the ground and first excited states have been calculated from high-level ab initio potential energy surfaces using a variational method. The vibrational frequencies and anharmonicities have been derived from these energy levels and the boron and hydrogen isotope shifts have been predicted. Although the ground state energy levels are for the most part well behaved, the excited state levels show substantial Renner–Teller mixing with nearby ground vibrational states. The calculations in the present work have been successfully used in the companion paper to make vibrational assignments of the laser-induced fluorescence spectra of HBF and DBF. [ABSTRACT FROM AUTHOR]

Subsequent to our spectroscopic detection of the HBX (X=F, Cl, Br) free radicals (S.-G. He, F. X. Sunahori, and D. J. Clouthier, J. Am. Chem. Soc. 127, 10814 (2005)), the electronic spectrum of the à 2A″Π-X 2A′ system of the fluoroborane (HBF) radical in the 600–745 nm region has been studied in detail using the pulsed discharge jet technique. The band system involves a linear-bent transition between the two Renner–Teller components of what would be a 2Π state at linearity. Using the results of our theoretical study of the ground and excited state vibrational energy levels and 11B–10B isotope shifts (see the companion paper), the vibrational quantum numbers of the bands in the laser-induced fluorescence (LIF) spectra have been assigned. Rotational and vibrational analyses of the LIF and wavelength resolved emission spectra have been carried out, from which the linear excited state and the bent ground state equilibrium configurations have been confirmed. The ground state molecular geometry of HBF has been determined as r0(BH)=1.214(2) Å, r0(BF)=1.303 4(5) Å, and θ=120.7(1)°. Based on high-level ab initio calculations and symmetry considerations, predissociation of the excited state into H(2S)+BF(1Σ+) on the ground state potential energy surface is identified as the cause of the breaking off of fluorescence in the LIF spectra. [ABSTRACT FROM AUTHOR]

Eight isotopologues of HGeBr and nine of DGeBr have been studied in natural abundance by pulsed-jet Fourier transform microwave spectroscopy. The reactive germylene species were produced in an electric discharge at the exit of a pulsed molecular beam valve using precursor mixtures of H3GeBr or D3GeBr in high pressure neon. In the 5–25 GHz operating range of the spectrometer, only a-type transitions were observed; K=0 transitions for HGeBr and K=0 and 1 transitions for DGeBr. From the observed transitions, an improved molecular geometry has been determined and nuclear quadruple constants for Ge and Br have been determined. The Townes–Dailey model has been extended to obtain the electron densities of the 4p orbitals on the germanium and bromine atoms from the quadruple coupling constants. These results are discussed in terms of qualitative molecular orbital theory. [ABSTRACT FROM AUTHOR]

The 2Δr-X 2Πr band system of the jet-cooled arsenic dicarbide (C2As) free radical has been recorded by laser-induced fluorescence (LIF) techniques in the 685–588 nm region. The radical was produced in a pulsed electric discharge jet using a precursor mixture of AsCl3 vapor and methane in high pressure argon. A series of weak bands involving all three excited state vibrations was observed for both 12C2As and 13C2As. High-resolution spectra of the 2Π1/2 component of the 000 bands of both isotopomers were rotationally analyzed, leading to the conclusion that the upper state is 2Δ with a small spin-orbit splitting (A=2.78 cm-1). Ground and excited state molecular structures of r0″(CC,ab initio)=1.2933 Å, r0″(CAs)=1.734(4) Å and r0′(CC,ab initio)=1.2276 Å, r0′(CAs)=1.830(3) Å were derived from the B values and our density functional predictions of the C[Single_Bond]C bond lengths. Single vibronic level emission spectra were recorded for many of the LIF bands and these were used to obtain the ground state vibrational frequencies and spin-orbit splittings. These data were satisfactorily fitted to a Renner–Teller model which gave 12C2As parameters of [variant_greek_epsilon]=0.695(8), ω1=1704.8(20) cm-1, ω2=161.6(8) cm-1, ω3=663.6(12) cm-1, and a spin-orbit constant A=857.7(11) cm-1. [ABSTRACT FROM AUTHOR]

Subsequent to our spectroscopic detection of the C2X(X=P,As) free radicals [F. X. Sunahori et al., J. Am. Chem. Soc. 129, 9600 (2007)], we have studied the electronic spectrum of the 2Δi-X 2Πr system of the jet-cooled C2P free radical in the 490–630 nm region. The high-resolution laser-induced fluorescence spectrum of the two spin components of the 000 band of 12C2P has been recorded, and the rotational and spin-orbit coupling constants have been determined for both electronic states. The Renner–Teller effect has been observed in both the 2Π and the 2Δ states, and the vibrational structure has been assigned. For the ground state, all of the observed levels up to 3500 cm-1 were fitted with a standard Renner–Teller model. The excited 2Δ state vibrational levels were successfully fitted using literature energy level expressions derived from perturbation theory, yielding vibrational and Renner–Teller parameters for both 12C2P and 13C2P. The molecular structure of C2P in the ground and excited states has also been estimated and compared to ab initio calculations and the geometries of similar molecules. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence spectra of jet-cooled HCP+ and DCP+ have been obtained with the pulsed discharge technique using HCP/DCP and argon precursor mixtures. Transitions involving all of the excited state vibrations have been observed and a set of vibrational constants has been obtained. High-resolution spectra of the 2Π3/2 components of the 000 bands of both isotopomers have been recorded, and these spectra show resolved phosphorus hyperfine structure which allowed the determination of the excited state Fermi contact parameter. The B values were used to obtain the ground and excited state effective geometric parameters as r0″(CH)=1.077(2) Å, r0″(CP)=1.6013(3) Å, r0′(CH)=1.082(2) Å, and r0′(CP)=1.5331(3) Å. A Renner-Teller analysis of the ground state vibrational energy levels obtained from the literature was attempted. All of the observed levels of DCP+ and the majority of those of HCP+ were satisfactorily fitted with a standard Renner-Teller model, but three HCP+ levels showed large systematic deviations which could not be accommodated by reassignments or improvements in the Fermi resonance Hamiltonian. Further improvements in the theory or in the experimental data will be needed to resolve this discrepancy. [ABSTRACT FROM AUTHOR]