Electronic bands of scandium monocarbide, ScC, in the region 14 140-16 000 cm-1.

Scandium monocarbide molecules, ScC, have been prepared by the reaction of 532 nm laser-ablated Sc metal with acetylene or methane under supersonic jet-cooled conditions. Electronic spectra of Sc12C and Sc13C have been recorded in the region 14 140-16 000 cm-1 using laser-induced fluorescence, and about 40 bands of each isotopomer have been analyzed rotationally. Wavelength resolved emission spectra have been obtained for many of them. The results show that Sc12C has a 2Πi ground state, with a bond length of 1.952 Å. Its vibrational frequency and spin-orbit coupling constant are 648 cm-1 and -39.47 cm-1, respectively (631 cm-1 and -39.32 cm-1 in Sc13C). Lying 155.58 cm-1 above the X2Π3/2 level (154.72 cm-1 in Sc13C) is a 4Π5/2 level, the lowest spin-orbit component of a 4Πi state. The excited states at higher energy are very complicated. Bands from both the doublet and quartet spin manifolds are present, and there are strong doublet-quartet interactions which induce many nominally-forbidden bands violating the selection rule ΔS = 0. Eight excited electronic states have been recognized, including four 4Δ states. These 4Δ states represent four of the five 4Δ states from the electron configurations (C 2pσ)2 (C 2pπ)2 (Sc 3dδ)1 and (C 2pσ)1 (C 2pπ)2 (Sc 4sσ)1 (Sc 3dδ)1.

High resolution quantum cascade laser spectroscopy of the simplest Criegee intermediate, CH2OO, between 1273 cm-1 and 1290 cm-1.

The region 1273-1290 cm-1 of the ν4 fundamental of the simplest Criegee intermediate, CH2OO, has been measured using a quantum cascade laser transient absorption spectrometer, which offers greater sensitivity and spectral resolution (<0.004 cm-1) than previous works based on thermal light sources. Gas phase CH2OO was generated from the reaction of CH2I + O2 at 298 K and 4 Torr. The analysis of the absorption spectrum has provided precise values for the vibrational frequency and the rotational constants, with fitting errors of a few MHz. The determined ratios of the rotational constants, A'/A″ = 0.9986, B'/B″ = 0.9974, and C'/C″ = 1.0010, and the relative intensities of the a- and b-type transitions, 90:10, are in good agreement with literature values from a theoretical calculation using the MULTIMODE approach, based on a high-level ab initio potential energy surface. The low-K (=Ka) lines can be fitted extremely well, but rotational perturbations by other vibrational modes disrupt the structure for K = 4 and K ≥ 6. Not only the spectral resolution but also the detection sensitivity of CH2OO IR transitions has been greatly improved in this work, allowing for unambiguous monitoring of CH2OO in kinetic studies at low concentrations.

Spectroscopic characterization of isomerization transition states.

Transition state theory is central to our understanding of chemical reaction dynamics. We demonstrate a method for extracting transition state energies and properties from a characteristic pattern found in frequency-domain spectra of isomerizing systems. This pattern-a dip in the spacings of certain barrier-proximal vibrational levels-can be understood using the concept of effective frequency, ω(eff). The method is applied to the cis-trans conformational change in the S1 state of C2H2 and the bond-breaking HCN-HNC isomerization. In both cases, the barrier heights derived from spectroscopic data agree extremely well with previous ab initio calculations. We also show that it is possible to distinguish between vibrational modes that are actively involved in the isomerization process and those that are passive bystanders.

Rotational analysis of bands of the à - X̃ transition of the C3Ar van der Waals complex.

Rotational analyses have been carried out for four of the strongest bands of the Ã-X̃ transition of the C3Ar van der Waals complex, at 393 and 399 nm. These bands lie near the 02(-)0-000 and 04(-)0-000 bands of the Ã(1)Πu-X̃(1)Σ(+) g transition of C3 and form two close pairs, each consisting of a type A and a type C band of an asymmetric top, about 4 cm(-1) apart. Only K″ = even lines are found, showing that the complex has two equivalent carbon atoms (I = 0), and must be T-shaped, or nearly so. Strong a- and b-axis electronic-rotational (Coriolis) coupling occurs between the upper states of a pair, since they correlate with a (1)Πu vibronic state of C3, where the degeneracy is lifted in the lower symmetry of the complex. Least squares rotational fits, including the coupling, have given the rotational constants for both electronic states: the van der Waals bond lengths are 3.81 and 3.755 Å, respectively, in the ground and excited electronic states. For the ground state our new quantum chemical calculations, using the Multi-Channel Time-Dependent Hartree method, indicate that the C3 unit is non-linear, and that the complex does not have a rigid-molecule structure, existing instead as a superposition of arrowhead (↑) and distorted Y-shaped (Y) structures.

Probing cis-trans isomerization in the S1 state of C2H2 via H-atom action and hot band-pumped IR-UV double resonance spectroscopies.

We report novel experimental strategies that should prove instrumental in extending the vibrational and rotational assignments of the S1 state of acetylene, C2H2, in the region of the cis-trans isomerization barrier. At present, the assignments are essentially complete up to ∼500 cm(-1) below the barrier. Two difficulties arise when the assignments are continued to higher energies. One is that predissociation into C2H + H sets in roughly 1100 cm(-1) below the barrier; the resulting quenching of laser-induced fluorescence (LIF) reduces its value for recording spectra in this region. The other difficulty is that tunneling through the barrier causes a staggering in the K-rotational structure of isomerizing vibrational levels. The assignment of these levels requires data for K values up to at least 3. Given the rotational selection rule K' - ℓ('') = ± 1, such data must be obtained via excited vibrational levels of the ground state with ℓ('') > 0. In this paper, high resolution H-atom resonance-enhanced multiphoton ionization spectra are demonstrated to contain predissociated bands which are almost invisible in LIF spectra, while preliminary data using a hyperthermal pulsed nozzle show that ℓ('') = 2 states can be selectively populated in a jet, giving access to K' = 3 states in IR-UV double resonance.

Reduced dimension rovibrational variational calculations of the S(1) state of C2H2. II. The S(1) rovibrational manifold and the effects of isomerization.

Reduced dimension variational calculations have been performed for the rovibrational level structure of the S1 state of acetylene. The state exhibits an unusually complicated level structure, for various reasons. First, the potential energy surface has two accessible conformers, trans and cis. The cis conformer lies about 2700 cm(-1) above the trans, and the barrier to cis-trans isomerization lies about 5000 cm(-1) above the trans minimum. The trans vibrations ν4 (torsion) and ν6 (asym. bend) interact very strongly by Darling-Dennison and Coriolis resonances, such that their combination levels and overtones form polyads with unexpected structures. Both conformers exhibit very large x36 cross-anharmonicity since the pathway to isomerization is a combination of ν6 and ν3 (sym. bend). Near the isomerization barrier, the vibrational levels show an even-odd K-staggering of their rotational levels as a result of quantum mechanical tunneling through the barrier. The present calculations address all of these complications, and reproduce the observed K-structures of the bending and C-C stretching levels with good qualitative accuracy. It is expected that they will assist with the assignment of the irregular patterns near the isomerization barrier.

Fluorescence lifetimes of the Ã1Πu state of C3.

The fluorescence lifetimes of 115 vibrational levels of the Ã1Π(u) state of C3 have been measured under supersonic molecular beam conditions. Of these, ninety-one are Π(u) vibronic levels, for which the lifetimes lie in the range 190-700 ns. The lifetimes of those Π(u) levels where only the bending vibration is excited lie in the range 190-235 ns. There is very little variation with bending quantum number, and the lifetimes of the two orbital components of the 1Π(u) state are essentially the same. When ν1 and ν3 are excited, the lifetimes become longer and/or reach a maximum for levels with v1 + v3 ~ 4. Excitation of the bending vibration in addition to the stretching vibrations shortens the lifetime slightly. Several of the levels show double-exponential decays. Another 23 levels, of Σ(u)+ vibronic symmetry, mostly have lifetimes that are longer than 300 ns. Interaction with nearby "dark" electronic states, such as B1Σ(u)-, B'1Δ(u), C1Π(g), and b3Π(g), is proposed to account for the observed lifetime lengthening. A particularly clear instance of such an interaction is the long lifetime (914 ns) of a perturbing Σ(u)+ level at 30,181 cm(-1), which is confirmed as belonging to the perturbing B'1Δ(u) state. A single level of Δ(u) symmetry at 29,170 cm(-1), which perturbs one of the Π(u) levels, is shown to belong to the à state.

Cis-trans isomerization in the S1 state of acetylene: identification of cis-well vibrational levels.

A systematic analysis of the S(1)-trans (Ã(1)A(u)) state of acetylene, using IR-UV double resonance along with one-photon fluorescence excitation spectra, has allowed assignment of at least part of every single vibrational state or polyad up to a vibrational energy of 4200 cm(-1). Four observed vibrational levels remain unassigned, for which no place can be found in the level structure of the trans-well. The most prominent of these lies at 46 175 cm(-1). Its (13)C isotope shift, exceptionally long radiative lifetime, unexpected rotational selection rules, and lack of significant Zeeman effect, combined with the fact that no other singlet electronic states are expected at this energy, indicate that it is a vibrational level of the S(1)-cis isomer (Ã(1)A(2)). Guided by ab initio calculations [J. H. Baraban, A. R. Beck, A. H. Steeves, J. F. Stanton, and R. W. Field, J. Chem. Phys. 134, 244311 (2011)] of the cis-well vibrational frequencies, the vibrational assignments of these four levels can be established from their vibrational symmetries together with the (13)C isotope shift of the 46 175 cm(-1) level (assigned here as cis-3(1)6(1)). The S(1)-cis zero-point level is deduced to lie near 44 900 cm(-1), and the ν(6) vibrational frequency of the S(1)-cis well is found to be roughly 565 cm(-1); these values are in remarkably good agreement with the results of recent ab initio calculations. The 46 175 cm(-1) vibrational level is found to have a 3.9 cm(-1) staggering of its K-rotational structure as a result of quantum mechanical tunneling through the isomerization barrier. Such tunneling does not give rise to ammonia-type inversion doubling, because the cis and trans isomers are not equivalent; instead the odd-K rotational levels of a given vibrational level are systematically shifted relative to the even-K rotational levels, leading to a staggering of the K-structure. These various observations represent the first definite assignment of an isomer of acetylene that was previously thought to be unobservable, as well as the first high resolution spectroscopic results describing cis-trans isomerization.

The C3-bending vibrational levels of the C3-Kr and C3-Xe van der Waals complexes studied by their Ã-X̃ electronic transitions and by ab initio calculations.

Fluorescence excitation spectra and wavelength-resolved emission spectra of the C(3)-Kr and C(3)-Xe van der Waals (vdW) complexes have been recorded near the 2(2-)(0), 2(2+)(0), 2(4-)(0), and 1(1)(0) bands of the Ã(1)Π(u)-X̃(1)Σ(g)(+) system of the C(3) molecule. In the excitation spectra, the spectral features of the two complexes are red-shifted relative to those of free C(3) by 21.9-38.2 and 34.3-36.1 cm(-1), respectively. The emission spectra from the à state of the Kr complex consist of progressions in the two C(3)-bending vibrations (ν(2), ν(4)), the vdW stretching (ν(3)), and bending vibrations (ν(6)), suggesting that the equilibrium geometry in the X̃ state is nonlinear. As in the Ar complex [Zhang et al., J. Chem. Phys. 120, 3189 (2004)], the C(3)-bending vibrational levels of the Kr complex shift progressively to lower energy with respect to those of free C(3) as the bending quantum number increases. Their vibrational structures could be modeled as perturbed harmonic oscillators, with the dipole-induced dipole terms of the Ar and Kr complexes scaled roughly by the polarizabilities of the Ar and Kr atoms. Emission spectra of the Xe complex, excited near the Ã, 2(2-) level of free C(3), consist only of progressions in even quanta of the C(3)-bending and vdW modes, implying that the geometry in the higher vibrational levels (υ(bend) ≥ 4, E(vib) ≥ 328 cm(-1)) of the X̃ state is (vibrationally averaged) linear. In this structure the Xe atom bonds to one of the terminal carbons nearly along the inertial a-axis of bent C(3). Our ab initio calculations of the Xe complex at the level of CCSD(T)∕aug-cc-pVTZ (C) and aug-cc-pVTZ-PP (Xe) predict that its equilibrium geometry is T-shaped (as in the Ar and Kr complexes), and also support the assignment of a stable linear isomer when the amplitude of the C(3) bending vibration is large (υ(4) ≥ 4).

IR-UV double-resonance of methyl radicals and a determination of the detection sensitivity of REMPI bands.

A novel method is exploited in this report to directly determine the relative detection sensitivity of the (2+1) resonance-enhanced multiphoton ionization (REMPI) bands of CH(3) and CHD(2) radicals. The basic idea is based on the simple fact that in an infrared (IR) absorption process the number of molecules being pumped from the lower state must be the same as the number of molecules in the excited upper state. Hence, the measured intensities of the respective REMPI bands should directly reflect their relative detection sensitivities. In order to ensure the processes involved and better quantify the measurements, extensive IR-UV double resonance spectroscopy was also performed. Using the REMPI-IR scheme, the IR spectrum of the v(1) fundamental (CH stretch) of CHD(2) was obtained and assigned for the first time. Using the IR-REMPI approach, high-resolution (2+1) REMPI spectra via the Rydberg 3p states of both radicals were demonstrated in a rotationally specific manner for both the origin and vibronic-excited bands, from which the predissociation rates of the Rydberg 3p states were deduced.

Darling-Dennison resonance and Coriolis coupling in the bending overtones of the A 1A(u) state of acetylene, C2H2.

Rotational analyses have been carried out for the overtones of the nu(4) (torsion) and nu(6) (in-plane cis-bend) vibrations of the A (1)A(u) state of C(2)H(2). The v(4)+v(6)=2 vibrational polyad was observed in high-sensitivity one-photon laser-induced fluorescence spectra and the v(4)+v(6)=3 polyad was observed in IR-UV double resonance spectra via the ground state nu(3) (Sigma(+) (u)) and nu(3)+nu(4) (Pi(u)) vibrational levels. The structures of these polyads are dominated by the effects of vibrational angular momentum: Vibrational levels of different symmetry interact via strong a-and b-axis Coriolis coupling, while levels of the same symmetry interact via Darling-Dennison resonance, where the interaction parameter has the exceptionally large value K(4466)=-51.68 cm(-1). The K-structures of the polyads bear almost no resemblance to the normal asymmetric top patterns, and many local avoided crossings occur between close-lying levels with nominal K-values differing by one or more units. Least squares analysis shows that the coupling parameters change only slightly with vibrational excitation, which has allowed successful predictions of the structures of the higher polyads: A number of weak bands from the v(4)+v(6)=4 and 5 polyads have been identified unambiguously. The state discovered by Scherer et al. [J. Chem. Phys. 85, 6315 (1986)], which appears to interact with the K=1 levels of the 3(3) vibrational state at low J, is identified as the second highest of the five K=1 members of the v(4)+v(6)=4 polyad. After allowing for the Darling-Dennison resonance, the zero-order bending structure can be represented by omega(4)=764.71, omega(6)=772.50, x(44)=0.19, x(66)=-4.23, and x(46)=11.39 cm(-1). The parameters x(46) and K(4466) are both sums of contributions from the vibrational angular momentum and from the anharmonic force field. For x(46) these contributions are 14.12 and -2.73 cm(-1), respectively, while the corresponding values for K(4466) are -28.24 and -23.44 cm(-1). It is remarkable how severely the coupling of nu(4) and nu(6) distorts the overtone polyads, and also how in this case the effects of vibrational angular momentum outweigh those of anharmonicity in causing the distortion.

The permanent electric dipole moment of chromium monodeuteride, CrD.

A number of low-N lines of the X (6)Sigma(+)<--A (6)Sigma(+)(0,0) band of chromium monodeuteride, CrD, have been recorded at near the natural linewidth limit by high resolution laser excitation spectroscopy of a supersonic molecular beam sample. The shifts and splitting of these lines caused by a static electric field have been analyzed to give the permanent electric dipole moments of the X (6)Sigma(+)(upsilon=0) and A (6)Sigma(+)(upsilon=0) states as 3.510(33) and 1.153(3) D, respectively. The dipole moment of the A (6)Sigma(+)(upsilon=0) state can be measured with higher precision because of some interesting near degeneracies in its level structure. The trends in the observed dipole moments for the first-row transition metal monohydrides are rationalized and compared with theoretical predictions.

High resolution laser induced fluorescence spectroscopy of the [18.8] 3Phi(i)-X 3Phi(i) (0,0) band of cobalt monofluoride.

The fine and hyperfine interaction parameters in the [18.8] (3)Phi (upsilon=0) and X (3)Phi (upsilon=0) states of cobalt monofluoride, CoF, have been determined from an analysis of high-resolution laser induced fluorescence spectra of the [18.8] (3)Phi(3)-X (3)Phi(3) and [18.8] (3)Phi(4)-X (3)Phi(4) band systems. The previously reported pure rotational transitions of the X (3)Phi(4)(upsilon=0) state [T. Okabayashi and M. Tanimoto, J. Mol. Spectrosc. 221, 149 (2003)] were included in the data set. The hyperfine parameters for (59)Co (I=72) and (19)F (I=12) have been interpreted using atomic data together with a proposed molecular orbital description for the [18.8](3)Phi(i) and X (3)Phi(i) states. A comparison of the hyperfine parameters in the X (3)Phi state of cobalt monohydride, CoH, with those of the X (3)Phi state of CoF reveals that the bonding in the two molecules is significantly different. It is shown that, in a situation where the Omega substates of a multiplet degenerate electronic state are analyzed separately, the Fermi contact parameter b can be determined with fair accuracy from the apparent centrifugal distortion of the hyperfine structure.

Low-N lines of the A6sigma+-X6sigma+ (1,0) band of CrH.

The (1,0) band of the A6sigma+-X6sigma+ electronic transition of CrH has been observed by laser-induced fluorescence following the reaction of laser-ablated Cr atoms with methanol under supersonic free-jet cooled conditions. Rotational assignments of the levels with N < or = 3 have been made by combination differences and dispersed fluorescence experiments on selected lines. These assignments complement those made from previously-recorded Fourier transform emission spectra, in which higher-N lines were assigned. The low-N rotational levels are extensively perturbed, presumably by levels of the a4sigma+, upsilon = 1 and B6pi, upsilon = 0 states.

The 4051-Angstroms band of C3 (A 1Piu-X 1Sigmag +, 000-000): perturbed low-J lines and lifetime measurements.

Rotational analyses have been carried out at high resolution for the 000-000 and 000-100 bands of the A (1)Pi(u)-X (1)Sigma(g) (+) transition of supersonic jet-cooled C(3). Two different spectra have been recorded for each band, using time gatings of 20-150 and 800-2300 ns. At the shorter time delay the spectra show only the lines observed by many previous workers. At the longer time delay many extra lines appear, some of which have been observed previously by [McCall et al.Chem. Phys. Lett. 374, 583 (2003)] in cavity ring-down spectra of jet-cooled C(3). Detailed analysis of these extra lines shows that at least two long-lived states perturb the A (1)Pi(u), 000 state. One of these appears to be a (3)Sigma(u) (-) vibronic state, which may possibly be a high vibrational level of the b (3)Pi(g) state, and the other appears to be a P = 1 state with a low rotational constant B. Our spectra also confirm the reassignment by McCall et al. of the R(0) line of the 000-000 band, which is consistent with the spectra recorded towards a number of stars that indicate the presence of C(3) in the interstellar medium. Fluorescence lifetimes have been measured for a number of upper-state rotational levels. The rotational levels of the A (1)Pi(u) state have lifetimes in the range of 230-190 ns, decreasing slightly with J; the levels of the perturbing states have much longer lifetimes, with some of them showing biexponential decays. An improved value has been obtained for the nu(1) vibrational frequency of the ground state, nu(1) = 1224.4933 +/- 0.0029 cm(-1).