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A new 4Σ − a4Πi electronic transition of ScC with (0,0) band near 12,700 cm−1 has been discovered using laser-induced fluorescence, following the reaction of laser-ablated scandium metal with acetylene or methane under supersonic jet-cooled conditions. The new bands are weak, since the a4Πi lower state is known from previous work to lie 160 cm−1 above the X2Π ground state; several spectra needed to be co-added in order to improve the signal-to-noise ratio. The rotational structure of the new transition is dense and heavily blended, but analysis has been possible for the strongest sub-bands of the (1,0) and (2,0) bands, respectively at 13,301 and 13,899 cm−1; extensive use was made of a band contour program for 4Σ − 4Π bands which listed the details of each line that it plotted. The bond length in the new 4Σ state is r0 = 1.964 A, which is 0.01 A longer than in the ground state; its vibrational frequency is 607 cm−1. There is no information in the spectrum giving the Kronig symmetry of the new state, though its electron configuration is probably (C 2pσ)1 (C 2pπ)2 (Sc 4sσ)1 (Sc 3dδ)1. Unexpectedly, the spin-spin interaction parameter, λ, in the new 4Σ state is negative, which suggests complicated interactions with nearby unseen electronic states.

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 Sc

The region 1273-1290 cm

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[superscript]eff. The method is applied to the cis-trans conformational change in the S[subscript 1] state of C[subscript 2]H[Subscript 2] 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., National Science Foundation (U.S.) (NSF Graduate Research Fellowship DGE 1144083), Alexander von Humboldt-Stiftung (Feodor Lynen fellowship), United States. Department of Energy (Grant DE-FG0287ER136)

We calculate second-order vibrational perturbation theory (VPT2) anharmonic force fields for the cis and trans conformers of S1 C2H2, and compare the results to experiment. The vibrational assignments of recently observed levels belonging to the cis well are of particular interest. A refined estimate of the cis origin position (44,870 ± 10 cm−1) is proposed, and preliminary low-energy fits to the global J = K = 0 trans level structure are also described. The performance of perturbation theory in this isomerizing system is examined, and both surprising successes and failures are encountered. We examine these and their causes, and offer practical suggestions for avoiding the pitfalls of applying perturbation theory to systems with large amplitude motions.

Author Institution: Optical Technology Division, NIST, Gaithersburg, MD 20899-8441, MD, USA; Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 10617 and Department of Chemistry, University of British Columbia, Vancouver, B.C., Canada V6T 1Z1

The vibrational structure of the A ˜ 1 Π u electronic state of C3 in the region 26 000–30 775 cm−1 has been re-examined, using laser excitation spectra of jet-cooled molecules. Rotational constants and vibrational energies have been determined for over 60 previously-unreported vibronic levels; a number of other levels have been re-assigned. The vibrational structure is complicated by interactions between levels of the upper and lower Born–Oppenheimer components of the A ˜ 1 Π u state, and by the effects of the double minimum potential in the Q 3 coordinate, recognized by Izuha and Yamanouchi [16] . The present work shows that there is also strong anharmonic resonance between the overtones of the ν 1 and ν 3 vibrations. For instance, the levels 2 1+ 1 and 0 1 + 3 are nearly degenerate in zero order, but as a result of the resonance they give rise to two levels 139 cm−1 apart, centered about the expected position of the 2 1+ 1 level. With these irregularities recognized, every observed vibrational level up to 30 000 cm−1 (a vibrational energy of over 5000 cm−1) can now be assigned. A Σ u + vibronic level at 30181.4 cm−1, which has a much lower B′ rotational constant than nearby levels of the A ˜ 1 Π u state, possibly represents the onset of vibronic perturbations by the B ˜ ′ 1 Δ u electronic state; this state is so far unknown, but is predicted by the ab initio calculations of Ahmed et al. [36] .

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 A. 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.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 A. 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 violatin...

Rotational analyses are reported for a number of newly-discovered vibrational levels of the S 1 - trans (A 1 A u ) state of C 2 H 2 . These levels are combinations where the Franck–Condon active ν 2 ′ and ν 3 ′ vibrational modes are excited together with the low-lying bending vibrations, ν 4 ′ and ν 6 ′ . The structures of the bands are complicated by strong a - and b -axis Coriolis coupling, as well as Darling–Dennison resonance for those bands that involve overtones of the bending vibrations. The most interesting result is the strong anharmonicity in the combinations of ν 3 ′ ( trans bend, a g ) and ν 6 ′ (in-plane cis bend, b u ). This anharmonicity presumably represents the approach of the molecule to the trans – cis isomerization barrier, where ab initio results have predicted the transition state to be half-linear, corresponding to simultaneous excitation of ν 3 ′ and ν 6 ′ . The anharmonicity also causes difficulty in the least squares fitting of some of the polyads, because the simple model of Coriolis coupling and Darling–Dennison resonance starts to break down. The effective Darling–Dennison parameter, K 4466 , is found to increase rapidly with excitation of ν 3 ′ , while many small centrifugal distortion terms have had to be included in the least squares fits in order to reproduce the rotational structure correctly. Fermi resonances become important where the K -structures of different polyads overlap, as happens with the 2 1 3 1 B 1 and 3 1 B 3 polyads ( B = 4 or 6). The aim of this work is to establish the detailed vibrational level structure of the S 1 - trans state in order to search for possible S 1 - cis ( 1 A 2 ) levels. This work, along with results from other workers, identifies at least one K sub-level of every single vibrational level expected up to a vibrational energy of 3500 cm −1 .

Perturbations in the K = 1 and 3 levels of the [Formula: see text], 4ν3 state of acetylene are explained as interactions with levels of the [Formula: see text], 31B4 (v3 = 1, v4 + v6 = 4) polyad. A satisfactory least-squares fit to the perturbed level structure has been obtained, treating the 4ν3 state as an asymmetric top perturbed by isolated K = 1 and 3 levels. Accurate deperturbed rotational constants for the interacting states are presented.

Rotational analyses are reported for the and bands of the transition of C2H2 near 45,000 cm−1 (+2800 cm−1 relative to T 0) from jet-cooled laser-induced fluorescence spectra. While the band is unperturbed and straightforward to assign, the 11 level is strongly perturbed by interactions with the 21 B 2 polyad, where υ B ′ = υ4′ + υ6′. In order to assign the lines of this band, a population-labelling technique was used, employing an infrared laser to deplete the population in selected ground state rotational levels before probing with the ultraviolet laser. Deperturbation of the 11/21 B 2 interaction leads to the value cm−1 for the fundamental symmetric C–H stretching frequency. Assignments are also reported for the 23 and 1121 levels, completing all assignments of levels containing excitation in only the totally symmetric vibrational modes up to +4500 cm−1. The reassignment of implies that some of currently accepted assignments above 47,000 cm−1 are in error and suggests that the interpretation of some asp...

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.

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.

Electronic band systems of zirconium monocarbide, ZrC, in the 16 000–19 000 cm−1 region have been observed following the reaction of laser-ablated Zr atoms with methane under supersonic free-jet conditions. Rotational analyses of high-resolution spectra have shown that the ground state of ZrC is a 3Σ state, with r0=1.8066 A and an unexpectedly small spin–spin parameter, λ=0.5139 cm−1. The spectra are dense because of the five naturally occurring isotopes of Zr. Four of these, with mass numbers 90, 92, 94, and 96, have I=0, but the fifth, 91Zr, present in 11.22% abundance, has I=5/2. Lines of 91ZrC can be assigned in some of the strongest bands, and are found to display sizeable hyperfine splittings, with widths of up to 0.2 cm−1. Analysis shows that the largest hyperfine effects are in the ground state, where b=−0.03133±0.00015 cm−1 and c=−0.00123±0.00037 cm−1 (3σ error limits). The large Fermi contact parameter, b, indicates that an unpaired Zr 5sσ electron is present, which, taken together with the small value of λ, means that the ground state must be a 3Σ+ state, from the electron configuration (Zr 5sσ)1 (C 2pσ)1. Internal hyperfine perturbations occur between the F1 and F3 electron spin components of the ground state in the range N=2–4, producing extra lines in some of the branches; the perturbations are of the type ΔN=0, ΔJ=±2, and are a second-order effect arising because the F1 (J=N+1) and F3 (J=N−1) spin components both interact with the F2 (J=N) component through ΔN=0, ΔJ=±1 matrix elements of the Fermi contact operator. Second-order perturbations of this type can only occur in states that are very close to case (b) coupling.

Rydberg states of calcium monofluoride in the n* = 17–20 region have been observed by ionization-detected optical–optical double-resonance spectroscopy via the D2Σ+ v = 1 intermediate state. All members of the six core-penetrating Rydberg series in the n* = 17–20 region and several components of the 17f and 17g core-nonpenetrating Rydberg states have been assigned. While the assignment of core-penetrating Rydberg states is straightforward without use of an effective Hamiltonian model, "spectrum-only" assignment of core-nonpenetrating states is complicated because strong l-uncoupling causes the core-nonpenetrating states to evolve rapidly from Hund's case (b) to Hund's case (d) coupling. We describe "spectrum-only" assignment procedures, developed in the spirit of Gerhard Herzberg, that can be used to assign optical–optical double-resonance spectra of core-penetrating and core-nonpenetrating Rydberg states using only information contained in the spectrum rather than predictions derived from an effective Hamiltonian model. The ambiguities that arise in the assignment of each class of states are discussed in detail.Key words: CaF, electric quadrupole moment, Rydberg states, laser spectroscopy.

The vibrational energy levels of the jet-cooled GeCH and GeCD radicals have been studied by a combination of laser-induced fluorescence and wavelength-resolved emission techniques. The radicals were produced in a pulsed electric discharge free jet expansion using methyltrichlorogermane and tetramethylgermane-d12 precursors. A re-examination of the weaker hot bands in the LIF spectrum has provided a more complete vibrational analysis of the upper state energy levels. The single vibronic level emission spectra obtained by pumping several bands of each isotopomer were analyzed to provide information on the low-lying ground state vibronic energy levels up to 3000 cm−1 above the zero-point level. Strong interactions occur in these molecules between vibronic levels with the same value of P, but differing by one unit of v2; such interactions were first described for NCS by Northrup and Sears [Mol. Phys. 71, 45 (1990)] and are conveniently called “Sears resonances.” The spectra of GeCH were further complicated by...

The (0,0) and (0,1) bands of the C4Σ−–X4Σ− electronic transition of VS (near 809 and 846 nm, respectively) have been recorded at high resolution by laser-induced fluorescence, following the reaction of laser-ablated vanadium atoms with CS2 under supersonic free-jet conditions. A least squares fit to the resolved hyperfine components of the rotational lines gives the rotational constants and bond lengths as C4Σ−: B 0 =0.188898±0.000027 cm −1 , r 0 =2.1315 A ; X4Σ−: B 0 =0.203684±0.000025 cm −1 , r 0 =2.0526 A . The electron spin parameters for the two states show that there are some similarities between the states of VS and those of VO, but the hyperfine parameters show that the compositions of the partly filled molecular orbitals are by no means the same. The ground state Fermi contact parameter of VS, b(X4Σ−), is only 58% of that of the ground state of VO, which implies that the σ orbital of the ground σδ2 electron configuration has less than 50% vanadium 4s character. Similarly, the excited state Fermi contact parameter, b(C4Σ−), is very much smaller than that of VO. No local rotational perturbations have been found in the C4Σ− state of VS, though an internal hyperfine perturbation between the F2 and F3 electron components at low N confuses the hyperfine structure and induces some forbidden (ΔJ=±2) rotational branches.

New laser-induced fluorescence spectra of supersonic jet cooled acetylene (C2H2) in the wavelength region 230–205 nm have led to an improved understanding of the vibrational structure of the A 1Au state. Among the new bands observed are two weak perturbed bands at 46008 cm−1 and 46116 cm−1. Rotational analyses of these bands, together with the corresponding ‘hot’ bands arising from the ground state v4 fundamental, have shown that the upper states have asymmetric top K structure that is unaffected by a axis Coriolis coupling; this means that they do not involve overtones of the low frequency bending vibrations and therefore must be combinations of ag vibrational normal modes. From their positions in the manifold, their vibrational assignments can only be 22 031 0; and 11 0;31 0. These assignments lead to values of x 22, x13, and a revised value for the symmetric CH stretching frequency, ν1 = 2880.5cm−1; this revised value is 160cm−1 lower than the previously accepted value, but consistent with new ab initi...

The optical Stark effect has been measured for the B 2Σ+−X 2Σ+, 301, and 201(P′=3/2) bands of lanthanum imide, LaNH, prepared in a supersonic molecular beam. The permanent electric dipole moment, μ, for the X 2Σ+ state was determined to be 2.53±0.06 D from measurements of the second-order Stark shifts in the B 2Σ+−X 2Σ+, 301 band. (ν3 is the La–N stretching vibration, of symmetry σ+.) The dipole moment in the B 2Σ+ state is too small to be determined from the Stark shifts in this band, but could be determined from the first-order Stark splittings in the P′=3/2 component of the B 2Σ+, 21 bending fundamental (of upper state π symmetry); it was found to be 0.3±0.2 D. A comparison is made with LaO and other isovalent molecules involving Group IIIA atoms.

The 000–000 and 310 bands of the 775-nm electronic transition of YC2(A2A1←X2A1) have been studied at high resolution, using the laser-induced fluorescence from a supersonic jet expansion. Three types of experiment have been carried out. First, the complete rotational and hyperfine structures of the two bands were recorded. To measure the small asymmetry splittings in the K=2 levels of the X2A1 state, portions of the b-type 310 band were then recorded in the presence of a weak static electric field. Finally, a number of pure rotational transitions between the K=0 levels of the ground state were recorded by pump/probe microwave optical double resonance. A few small rotational perturbations occur in the upper electronic state but, omitting the perturbed lines, the combined data sets could be modeled using an effective Hamiltonian operator appropriate for the rotation, electron spin, and hyperfine structure of a rigid asymmetric top molecule. The molecule is confirmed as being “T-shaped,” where the Y atom is bonded to the side of a C2 group; the rotational constants determined are for the A2A1, 31 level, A=1.76128, B=0.189949, C=0.170056 cm−1, and for the X2A1, v=0 level, A=1.742731, B=0.201947, C=0.181285 cm−1. Allowing for electron orbital corrections to the rotational constants, the geometrical structures are found to be A2A1 state, r (Y–C)=2.2795 A, r (C–C)=1.2630 A, ∠C–Y–C=32.17°; X2A1 state, r (Y–C)=2.1946 A, r (C–C)=1.2697 A, ∠C–Y–C=33.63°. A molecular orbital diagram is given for the states of YC2 and the interpretation of the electron spin and hyperfine parameters is discussed.

B 2 � 3/2, ˜ C 2 � 7/2, and ˜ D 2 � 5/2, are derived from the configuration Ni + (3d 8 4s )C N − . Rotational analysis of bands of NiC 14 N and NiC 15 N at high resolution has given the bond lengths in the ˜ X1 2 � 5/2 ground state as r0 (Ni‐C) = 1.8292 ± 0.0028 A ❛ ; r0 (C‐N) = 1.1591 ± 0.0029 A ❛ . The orbital angular momentum splits the bending fundamental of the ˜ X1 2 � 5/2 state into two vibronic components, which lie at 243.640 cm −1 ( 2 � 3/2) and 244.964 cm −1 ( 2 � 7/2). Exceptionally strong Fermi resonance occurs in the ground state between the Ni‐C stretching vibration, ν3, and the overtone of the bending vibration. Sixty vibrational levels of the ground state with |l |= 0 and 1 have been assigned. They could be fitted by least squares to a simple matrix representation of the Fermi resonance that ignores the orbital angular momentum; the interaction matrix element, as a fraction of the bending frequency, turns out to be larger than that in the “prototype” molecule, CO2. The two low-lying excited electronic states, ˜ X2 2 � 3/2 at 830 cm −1 and ˜ W1 2 � 3/2 at 2238 cm −1 , have very similar properties to the ground state. The energies of the excited states in the visible region bear a remarkable resemblance to those found in NiH (S. A. Kadavathu et al., 1991, J. Mol. Spectrosc. 147, 448‐470). Again, the effects of Fermi resonance in them are large but those of vibronic coupling are surprisingly small. The most significant vibronic coupling occurs between the ˜ A 2 � 5/2 and ˜ B 2 � 3/2 states, which are separated by 79 cm −1 , an interval which is less than half the frequency of the bending vibration that couples them; large numbers of vibronically induced bands appear in the excitation and dispersed emission spectra, but the splitting between the vibronic components of the bending fundamental of the ˜ A 2 � 5/2 state is only 6.988 cm −1 . Rotational perturbations are widespread in the ˜ A 2 � 5/2 state, particularly in the levels of the ν3 (Ni‐C stretching) progression. Vibrational assignments have been made on the basis of the 58 Ni‐ 60 Ni isotope shifts at high resolution and the Franck‐Condon patterns in dispersed emission spectra. Various weak unassigned bands, both in the excitation and the dispersed emission spectra, give evidence for the existence of at least three more excited electronic states which it has not been possible to characterize in detail. C � 2002 Elsevier Science (USA)

The rotationally resolved A (010)– X (000) spectrum of DCCl between 12 880 and 12 964 cm −1 was measured using frequency-modulated laser absorption spectroscopy of jet-cooled and ambient temperature samples. Transitions to levels with K a ′=0 and 1 were assigned, and their analysis leads to improved accuracy of both the ground state rotational constants of DCCl and, when combined with existing data for HCCl, the geometry of the radical. In addition to the expected perpendicular band structure, a number of parallel (Δ K a =0) subbands were observed. Their intensity derives from a combination of c -type Coriolis coupling and axis-switching (J. T. Hougen and J. K. G. Watson, 1965, Can. J. Phys. 43, 298) resulting from the change in geometry between the two states. The two contributions have been calculated for the (010)–(000) band of DCCl and previously recorded data for HCCl. Satisfactory agreement with experimental measurements was obtained. The Coriolis contributions are small for these transitions, but may add to or subtract from the axis-switching. For higher bending excitation in the upper state, Coriolis coupling is predicted to make larger contributions to the parallel subband intensities.

The (0,0) and (1,0) bands of the E 3 Π– X 3 Δ transition of TiO in the near-infrared have been recorded by frequency modulated laser absorption spectroscopy in a laser ablation/free jet expansion source. The observed linewidths (FWHM) varied from 300 to 500 MHz according to the expansion conditions and are dominated by residual Doppler broadening in the unskimmed source. Data for the (0,0) band have been obtained for TiO molecules containing all the naturally occurring Ti isotopes but, for the weaker (1,0) band, only for 48 TiO. Rotational constants for the two upper state vibrational levels were derived by fitting the data to an effective Hamiltonian; equilibrium parameters have been calculated. The experimental results are compared to the results of ab initio calculations on the E – X system. Ab initio results for the b – a system and for the lowest 3 Σ − state are also presented. They indicate that the D 3 Σ − state is not a very low-lying state.

High-resolution emission spectra of VO have been recorded in the region 3400–19 400 cm −1 using a Fourier transform spectrometer. The molecules were observed from the reaction of VOCl 3 with active nitrogen. Two new bands, with origins near 5539.46 and 5551.69 cm −1 , are assigned as the 0–0 bands of the 2 Φ 5/2 – 2 Δ 3/2 and 2 Φ 7/2 – 2 Δ 5/2 spin–orbit components (respectively) of a 2 Φ–1 2 Δ electronic transition of VO. A rotational analysis of both subbands has been carried out and spectroscopic constants have been extracted. The 1 2 Δ state is known from the previous analysis of the near infrared doublet transitions of VO and the new 2 Φ excited state has rotational constants very similar to those of another 2 Φ state observed previously [Merer et al. , J. Mol. Spectrosc. 125, 465 (1987)].

The region 1273-1290 cm$^{-1}$ of the $��$4 fundamental of the simplest Criegee intermediate, CH$_2$OO, has been measured using a quantum cascade laser transient absorption spectrometer, which offers greater sensitivity and spectral resolution (< 0.004 cm$^{-1}$) than previous works based on thermal light sources. Gas phase CH$_2$OO was generated from the reaction of CH$_2$I + O2 at 298 K and 4 Torr. 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 $\geq$ 6. Not only the spectral resolution but also the detection sensitivity of CH$_2$OO IR transitions has been greatly improved in this work, allowing for unambiguous monitoring of CH$_2$OO in kinetic studies at low concentrations.

A new band at 23 390 cm−1 in the A ˜ 1Πu– X ˜ 1 Σ g + electronic transition of C3 has been recorded by laser-induced fluorescence at high resolution and rotationally analyzed. Based on the agreement of the lower state constants with infra-red data given by Kawaguchi et al. [9] , the vibrational assignment is found to be 0 1 1(Δu)–0 1 1 (Πg). Seventeen of the 19 vibronic levels of the A ˜ 1Πu state expected to lie below 25 700 cm−1 (a vibrational energy of 1025 cm−1) have now been identified.

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.

Spectra of a 3B1(ν1ν2ν3)=(110) and (200)–X 1A1(000) of 32SO2 and 34SO2 are obtained in a supersonic jet at a resolution 0.015 cm−1. The rotational structures are analyzed for both isotopic species to obtain precise rotational and spin constants. Two new bands at 27 032.222(1) and 27 515.41(1) cm−1 arising from vibronic interaction separately with vibrational state (110) and (200) of a 3B1 are assigned to transitions to b 3A2(011) and (101). The rotational structure of b 3A2 is observed and analyzed for the first time. The two vibrational states are analyzed to yield effective rotational constants A=1.4348(3), 1.89(3), B=0.3610(3), 0.412(3), and C=0.2689(3), 0.239(3) cm−1, respectively. Then the difference ν1−ν2 of vibrational frequency of b 3A2 is obtained to be 483.19 cm−1. Because of interaction to b 3A2(011) the observed spacing of K stack and spin constants α and β for a 3B1(110) are smaller whereas the state a 3B1(200) is relatively unaffected and only spin constants vary significantly.

Laser-induced excitation spectra of the two bands a(3)B(1)-;X(1)A(1), 2(1)(0) and 1(1)(0) of (32)SO(2) and (34)SO(2) have been recorded in a supersonic jet at a resolution of 0.015 cm(-1). The rotational and electron-spin fine structure has been analyzed for both isotopic species. Analysis of the rotational and electron-spin fine structure yields precise values of the rotational constants A, B, and C and the spin constants alpha and beta for both (32)SO(2) and (34)SO(2) in the states a(3)B(1) (010) and (100). No interaction between these two vibrational states with any nearby triplet state is observed for rotational levels with J

The A 2Σ+–X 2Πi electronic transition of jet-cooled germanium methylidyne has been recorded in the 730–555 nm region by laser-induced fluorescence techniques. The radical was produced in a pulsed electric discharge using tetramethylgermane as the precursor. The band system consists of perpendicular and vibronically induced parallel bands with upper state frequencies of ν2′=638/497 cm−1 and ν3′=990/952 cm−1 for GeCH/GeCD. The 2Σ+–2Π3/2 components of the 000 bands of GeCH and GeCD and the 201 band of GeCH have been recorded with sufficient resolution to assign the transitions of the 70Ge, 72Ge, 74Ge, and 76Ge isotopomers. The rotational constants of the various hydrogen and germanium isotopomers were used to derive the following molecular structures: r0″(Ge–C)=1.775 79(21) A, r0″(C–H)=1.0673(12) A, r0′(Ge–C)=1.673 66(8) A, and r0′(C–H)=1.0589(5) A. The ground state germanium–carbon bond length is typical for a Ge=C double bond, but in the excited state it corresponds to a triple bond. This is a consequence...

The A 2Σ+–X 2Πi transition of jet-cooled silicon methylidyne, SiCH, has been recorded by laser-induced fluorescence in the 850–600 nm region. The radical was produced in an electric discharge using tetramethylsilane as the precursor. Fifteen cold bands of SiCH and 16 bands of SiCD have been assigned vibrationally, giving the upper state frequencies as v2′=715/558 cm−1 and v3′=1168/1127 cm−1 for SiCH/SiCD. Rotational analysis of the 000 and 303 bands for each isotopomer has given the following molecular structures: r0″(Si–C)=1.692 52(8), r0″(C–H)=1.0677(4), r0′(Si–C)=1.6118(1), and r0′(C–H)=1.0625(5) A. The silicon–carbon bond length in the X 2Π ground state of SiCH (electron configuration …σ2π3) is typical for a Si=C double bond; in the A 2Σ+ excited state (…σ1π4) it corresponds to a triple bond. This work provides the first experimental measurement of the length of the carbon–silicon triple bond.

Laser excitation of isotopically labeled SO2 molecules in a neon matrix has allowed the measurement of dispersed phosphorescence spectra (a3B1-X1A1) in the wavelength region 387.5−526.3 nm (19 000−25 800 cm-1). The origin of the a−X transition lies at 25 747 cm-1 in 16OS16O, shifted 18 cm-1 to the red from the gas phase; the three vibrational frequencies of the X state are nearly unperturbed in the neon matrix (1149, 520, and 1362 cm-1). As the zero-phonon lines are sharp and well resolved, it has been possible to monitor emission from a specific isotopomer to obtain its excitation spectrum. The excitation spectra of matrix-isolated 16OS16O and 18OS18O in the region 350.9−388.5 nm (25 740−28 500 cm-1) are similar to those previously observed in the gas phase, with strong perturbations affecting all vibrational levels above a3B1(110) at 27 005 cm-1 (370 nm). Two additional groups of bands are observed for the unsymmetrical isotopomer 18OS16O; they are identified as transitions to levels of the a3B1 stat...

The first high-resolution infrared spectra of the unstable molecule vinyl alcohol are reported. The spectra have been obtained using a new precursor, 2-chloroethanol, which when pyrolyzed at 1050 degrees C gives strong infrared spectra of vinyl alcohol free of interfering hydrocarbon absorption bands. In this work, we have analyzed the strong nu(13) fundamental at 817 cm(-1) and substantially improved the ground state rotational constants by a simultaneous fitting of previous microwave data and a large number of infrared combination differences. The 13(1) upper state was found to be perturbed by the 15(2) "dark" state at 775.7 cm(-1), and a complete analysis of the perturbed rotational structure has been achieved using an interacting two-state model. Further small perturbations at high K(a) and J have been identified as interactions with the nu(10) and nu(12) fundamentals at 948 and 960 cm(-1), respectively. Copyright 1999 Academic Press.

Laser excitation spectra of yttrium monohydroxide, YOH, have been recorded in the 500-625 nm wavelength region. Rotational analysis of bands of YOH and YOD has shown that the ground state is X1Sigma+, with the structure r0(Y-O) = 1.9486 A, r0(O-H) = 0.9206 A; the bending frequency (nu2, pi) is 313.73 cm-1 (237.43 cm-1 in YOD) and the Y-O stretching frequency (nu3, varsigma+) is 673.83 cm-1 (655.34 cm-1). Two excited electronic states have been identified; they are assigned as B1Pi (16 449 cm-1) and C1Sigma+ (18 509 cm-1). Unusually strong vibronic coupling through the bending vibration occurs between these two states, which causes their vibrational structures to be highly irregular; assignments have only been possible following extensive wavelength-resolved fluorescence experiments. The vibronic coupling raises the bending frequency of the C1Sigma+ state to 457 cm-1 and reduces that of the lower Born-Oppenheimer component of the B1Pi state (which has A' symmetry in the Cs point group) to the extent that the molecule becomes nonlinear, with a potential barrier at the linear configuration of about 120 cm-1. The presence of the potential barrier is clearly demonstrated by the level structure of YOD, where the Sigma+ vibronic component of the 010 vibrational level (linear molecule notation) lies 1.4 cm-1 below the 000 level. The upper Born-Oppenheimer component, which has A" symmetry, is unaffected; its bending frequency is similar to that of the ground state. Perturbations occur in both the B1Pi and C1Sigma+ states; some of these represent local interactions between the two of them, but others are caused by higher vibrational levels of lower-lying "dark" electronic states. Over 40 ground state vibrational levels have been identified for both YOH and YOD from the wavelength-resolved fluorescence spectra. Copyright 1999 Academic Press.

Laser excitation spectra of gaseous CoO have been recorded using Doppler-limited intracavity spectroscopy and supersonic jet-cooled molecular beam methods. As seen in the molecular beam spectra there are nearly 100 bands arising from the X4Delta7/2 spin component of the ground state in the wavelength region 430-720 nm. All of them are very strongly red-degraded and most of them are perturbed. 59Co hyperfine broadenings or splittings have been observed in many. The bands can be arranged into five electronic transitions, together with a considerable number of "extra" bands induced by perturbations; two of the excited electronic states are 4Phi, while the other three, which lie within 1700 cm-1, are 4Delta. Semi-empirical calculations have been carried out for the 4Delta excited states, using exchange integrals transferred from the TiO and VO spectra; the predicted energies, spin-orbit structures, and relative intensities in absorption are consistent with the assignment of all three 4Delta states to the same electron configuration, (4ssigma)1(3ddelta)3(3dpi)2(3dsigma)1. It seems that there are extensive interactions between the excited 4Delta states, because only one of them gives rise to a recognizable, though irregular, long vibrational progression in absorption. The other two progressions die out abruptly and unexpectedly after three or four members, presumably as a result of interference effects; however, wavelength-resolved fluorescence studies, together with calculations of the Franck-Condon overlap integrals to the various vibrational levels of the ground state, show that the upper state progressions reappear following a region of confused absorption. All of the excited states show highly irregular variations of their vibrational intervals and rotational constants with the vibrational quantum number, v. The wavelength-resolved fluorescence spectra give evidence for new low-lying electronic states of CoO at 3038, 5989, and 9105 cm-1; based on its vibrational frequency the 3038 cm-1 state is possibly the sigmadelta4pi2 B4Sigma- state. Copyright 1997 Academic Press. Copyright 1997Academic Press

The yttrium dicarbide molecule, YC2, has been identified in the reactions of laser-ablated yttrium with methane and other small hydrocarbons under supersonic jet-cooled expansion conditions. A parallel-polarized electronic band system with origin at 12 889.5 cm−1 has been tentatively identified as an A1–X 2A1 transition of a “T-shaped’’ isomer where the Y atom is bonded to the side of a C2 molecule. Weak vibronically induced perpendicular bands, representing Δυ3=odd transitions, are also present. Extensive vibrational progressions in the excitation and the resolved fluorescence spectra have been analyzed to give the two lowest vibrational frequencies of the two electronic states: X 2A1, ν2 (a1)=561.1(±2.8 cm−1), ν3 (b2)=369.6 (±1.7 cm−1); excited A1, ν2=494.5(±5.0 cm−1), ν3=331.6(±2.2 cm−1). No bands involving the C–C stretching vibration (ν1,a1) have been found. The frequency of the ν3 vibration in the ground state indicates that the barrier to internal rotation of the C2 group against the Y atom is qu...

Laser excitation spectra of TiO have been recorded in the 30 500–34 500 cm −1 energy region using the technique of optical–optical double resonance. Twenty 3 Φ and 24 3 Π excited vibrational levels have been observed in experiments that use the A 3 Φ and B 3 Π electronic states as intermediates. The new levels can be assigned to four electronic states, here labeled G 3 Φ, H 3 Φ, I 3 Π, and J 3 Π. All the excited states are found to be perturbed to some degree; estimates of the upper state constants have been obtained from least-squares fitting of the data. Laser-induced fluorescence to a new low-lying electronic state of TiO has been observed, following excitation to the I 3 Π and J 3 Π states; this state lies 12 284 ± 5 cm −1 above the X 3 Δ 1 , J = 1 ground rotational level of the molecule and is believed to be the hitherto unobserved δ 2 D 3 Σ − state.

Electronic bands assigned to the linear TiCH and TiCD radicals have been recorded by laser-induced fluorescence in the 620–725 nm wavelength region, following the reaction of laser-ablated Ti atoms with methane under supersonic jet-cooled molecular beam conditions. Rotational analyses of high-resolution spectra have given the ground state substitution structure X Σ + :r 0 (C–H) = 1.085 ± 0.002 A;r 0 (Ti–C) = 1.7277 ± 0.0003 A. The electronic upper state appears to be a very strongly perturbed2Π state, with many extra rotational and vibrational levels appearing; it has not been possible to make vibrational assignments, though the Ω′ values have been established for some of the levels. Wavelength-resolved fluorescence spectra have given two of the three ground state vibrational frequencies:TiCH: ν2(bend) = 578 cm−1, ν3(Ti–Cstretch) = 855 cm−1TiCD: ν2= 442 cm−1, ν3= 815 cm−1. Vibrationally forbidden transitions to the ground state bending fundamental appear prominently in many of these spectra, as a result of strong vibronic coupling. No evidence for ν1″(the C–H stretching vibration) has been found.

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.

The laser excitation spectrum of tungsten methylidyne, WCH, has been recorded in the 12 000–15 400 cm−1 region. A total of 14 vibronic bands of WCH and 16 bands of WCD have been observed in this region. Rotational analysis shows that the ground state is X3/2(2Δ), with the substitution structure r0(W≡C)=1.73665 A and r0(C–H)=1.0765 A. The excited vibronic levels have been assigned, on the basis of their WCH/WCD isotope shifts and their wavelength resolved fluorescence patterns, to three electronic states, A3/2(2Δ), B1/2(2Π), and C5/2(2Φ), at 12 090, 13 392, and 14 110 cm−1, respectively. The wavelength resolved fluorescence spectra have also established the low‐lying vibrational levels of the ground state. The ground state bending fundamental lies at 660 cm−1, while the W–C stretching frequency is 1006 cm−1; the corresponding frequencies in WCD are 501 cm−1 and 953 cm−1, respectively. No evidence for the C–H stretching frequency has been found. Emission has also been observed to a low‐lying electronic s...

High-resolution infrared spectra of the out-of-plane bending (ν 6 ) fundamentals of the transient molecules HCOCl and DCOCl are reported. The HCOCl band consists of a C -type component due to the out-of-plane transition moment from the ground state and a Coriolis-induced parallel component which derives its intensity from mixing of the 4 1 and 6 1 vibrational states. In DCOCl, the C -type component is absent, so that all of the band intensity comes from Coriolis-induced borrowing from ν 4 . The DCOCl ν 4 and ν 6 bands have been fitted simultaneously to account for the strong first-order and weaker second-order a -axis Coriolis interactions between the fundamentals. In HCOCl the ν 6 band is only weakly perturbed by ν 4 but is more strongly affected by local interactions with the 5 2 vibrational level, which have been explicitly included in the analysis. Ab initio predictions of the relative intensities of the ν 6 fundamentals show that the change in dipole moment is much smaller for DCOCl, accounting for the absence of the C -type component.

The laser induced fluorescence spectrum of jet‐cooled CoC near 750 nm has been measured at high resolution following the reaction of laser‐ablated cobalt atoms with methane. The X2Σ+ ground state of CoC is an unusually good example of Hund’s case (bβS) coupling. Since Co has a nuclear spin I=7/2, each rotational level is split by the Fermi contact interaction into G=3 and G=4 components, where G=I+S; the splitting for N=0 is more than 0.5 cm−1. The X2Σ+ state begins to uncouple toward case (bβJ) with increasing rotation. Transitions to various 2Π excited states occur in the region 13 000–14 500 cm−1; the most prominent of these (for which high resolution spectra have been recorded) lie at 13 079 cm−1 (2Π3/2) and 13 343 cm−1 (2Π1/2). The (bβS) coupling in the ground state produces some unexpected hyperfine intensity patterns, which have been studied in detail. A very low‐lying 2Δi state, whose Ω=5/2 and Ω=3/2 components lie at 221 and 1173 cm−1, has been identified. Laser excitation of the 2Π3/2–2Δ5/2 tran...

Three new bands of CoH and five new bands of CoD have been observed using laser excitation in the visible and near-infrared, following reaction of Co atoms with H 2 or D 2 under supersonic jet-cooled conditions. All the bands reported here, together with two reported previously by Varberg et al. ( J. Mol. Spectrosc. 138, 630-637, 1989), are shown to belong to a single electronic transition with an origin near 12 400 cm −1 , here designated A ′ 3 Φ 4 - X 3 Φ 4 . High-resolution spectra of the (0, 0) bands of both CoH and CoD reveal extensive hyperfine structure associated with the 59 Co nucleus, which shows that the electron configuration of the A ′ 3 Φ state is (7σ) 1 (3 d δ) 3 (3 d π) 3 (8 σ) 1 . Wavelength-resolved fluorescence from one of the CoD bands has given the Ω = 4-3 spin-orbit interval for the X 3 Φ state as 729 cm −1 and confirmed the presence of a low-lying Ω = 3 electronic state ∼ 2444 cm −1 above the ground state.

Gas-phase infrared spectra of the transient molecule sulfine, H 2 CSO, have been recorded in pyrolysis experiments using methanesulfinyl chloride or dimethyl sulfoxide as starting materials. Two strong vibrational fundamentals, ν 4 ( a ′, 1175 cm −1 ) and ν 8 ( a ″ 762 cm −1 ), have been observed at low resolution in the 1400-700 cm −1 region. The ν 8 band has been recorded at high resolution and a detailed analysis of the rotational structure has been carried out. Numerous excited state perturbations result from |Δ K a | = 2 A -type and |Δ K a | = 1 B -type Coriolis interactions with the 7 2 vibrational state at 784.2 cm −1 . Although 2ν 7 band transitions could not be directly observed, a complete analysis of the perturbed rotational structure has been possible, using a Hamiltonian which includes both types of Coriolis interactions. Accurate molecular constants have been obtained for both states with only two interaction constants, one for each mechanism. A few weak perturbations at very high K a values could not be fitted with this scheme and are assigned to high order Coriolis interactions with the 6 1 level at 866 cm −1 .

The orange system of FeO has been reinvestigated using low-temperature molecular beam laser-induced fluorescence spectra, obtained by supersonic jet cooling. Two new weak bands have been found, and analyses of some of the previously known bands extended. Measurements of the 54 Fe- 56 Fe isotope shifts have been made for most of the bands, and the hyperfine structure of the low- J lines has been recorded for two of the strongest bands of 57 FeO. The isotope shifts are consistent with the presence of two 5 Δ i - 5 Δ i transitions lying within 1000 cm −1 ; the origins of the Ω = 4 spin components lie at 5583 and 6110 A, respectively. The hyperfine patterns and the spin-orbit structure indicate that the upper state electron configurations are (3 d δ) 3 (3 d π) 2 (3 d σ) 1 , ( D 5 Δ i , 5583 A) and O(2 p π) 3 (4 s σ) 1 (3 d δ) 3 (3 d π) 3 , ( D ′ 5 Δ i , 6110 A). The bond length in the D ′ state ( r 0 = 1.654 A) has been obtained from a deperturbation of the 6110 A band; it is only 0.035 A longer than in the ground state, which indicates that electron promotion between the two π orbitals, nominally O(2 p π) and Fe(3 d π), has only a small effect on the strength of the bonding. The new isotope data still do not clarify the vibrational assignments of the higher levels, which are disorganized by extensive electronic perturbations.