Search

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.

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] .

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 .

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.

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.

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)].

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.

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 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

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.

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.

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.

The (0,0) band of the B4Π-X4Σ− system of VO near 7900 A has been analyzed from Doppler-limited Fourier transform emission spectra, together with wavelength-resolved laser-induced fluorescence and sub-Doppler intermodulated fluorescence spectra. The B4Π, ν = 0 level is heavily perturbed by the a2Σ+, ν = 2 level. Because the 3dδ2 4sσ a2Σ+ and 3dδ2 3dπ B4Π states have very different hyperfine parameters, the hyperfine patterns of the rotational lines vary dramatically with J at the perturbations. The observed hyperfine structure of the band, including the lowest-J perturbation, near J = 38 1 2 , could be explained in detail by a model Hamiltonian that takes the interactions into account: 3211 hyperfine line frequencies have been fitted with an rms error of 11 MHz. The b parameter (the coefficient of I·S in the magnetic hyperfine Hamiltonian) is well determined for the a2Σ+ state and provides the evidence for its electron configuration. The hyperfine structure of the B4Π state shows no effects of spin-orbit distortion and is well explained by the standard case (aβJ) model. Only the spin-orbit operator is needed to describe the interaction between the a and B states; the parameter for the direct dipolar interaction between them is too small to be determined. A fit to the higher-J rotational data showed that the spin-orbit interaction matrix element, which arises as a result of configuration interaction in the two states, has an apparent J dependence; this presumably means that the mixing is r dependent. Improved values for the ground state hyperfine parameters have been obtained from the new data, supplemented by the three very precise microwave frequencies measured by Suenram et al. (J. Mol. Spectrosc.148, 114, 1991).

The (1, 0) band of the B4Π-X4Σ− transition of VO has been analyzed rotationally, from Doppler-limited discharge emission and laser excitation spectra. The B4Π, v = 1 level is heavily perturbed by a vibrational level of the a2Σ+ state and, with the extra complexity caused by the 51V nuclear hyperfine structure, could be analyzed fully only by extensive wavelength-resolved laser-induced fluorescence experiments. Making use of data from the corresponding perturbations in B4Π, v = 0 and assuming that the vibrational dependence of the spin-orbit matrix element can be represented by a vibrational overlap integral, it has been possible to deduce the vibrational numbering of the perturbing a2Σ+ state without having to use isotopic data. It is found that the a2Σ+, v = 0 level lies near 10412 cm−1, and that B4Π, v = 1 is perturbed by a2Σ+, v = 3. The a2Σ+ state comes from the same electron configuration, (4sσ) (3dδ)2, as the X4Σ− ground state. The phases of the spin-orbit matrix elements between 4Π and 2Σ+ states must be chosen consistently in order to obtain a correct understanding of the rotational perturbations. A consistent set of phases is obtained with 〈 2 Σ + , F 2 | H SO | 4 Π 1 2 , f〉 = 〈 2 Σ + , F 1 | H SO | 4 Π 1 2 , e〉 = [formula] 〈 2 Σ + , F 2 | H SO | 4 Π − 1 2 , f〉 = −〈 2 Σ + , F 1 | H SO | 4 Π − 1 2 , e〉 = A/2, where A = 〈2Σ+ || Hspin-orbit || 4Π〉.

The first high-resolution (0.002 cm−1) infrared spectrum of the transient molecule formyl chloride, HCOCl, is reported. The ν3 type A/B hybrid band between 1220 and 1390 cm−1 has been analyzed in detail, providing assignments for the A- and B-type lines of both HCO35Cl and HCO37Cl. The ground state rotational constants have been refined by a simultaneous fitting of previously reported microwave data and combination differences from the ν3 and ν6 bands of HCOCl. The excited state constants of both isotopomers have been obtained from fitting assignments over a wide range of J and Ka values. By comparing relative intensities, we estimate the ratio of the B-type to A-type transition moments to be 0.25 ± 0.1. The ν3 band shows only minor perturbations.

Laser excitation spectra of the A ′ 5 Δ- X 5 Π (0,0) and (1.1) bands of CrO have been recorded in the region 11 750-12 000 cm −1 . Each band was found to consist of five distinct subbands. corresponding to the fise case (a) allowed spin components of a 5 Δ- 5 Π transition. The present data are combined with FTIR emission data for the A 5 Σ + - X 5 Π (0,0) band system recorded by Cheung et al. [ J. Mol. Spectrosc. 104, 315-336 (1984)], enabling the determination of improved molecular constants for the X 5 Π and A 5 Σ + states as well as the first set of reliable constants for the A ′ 5 Δ state. Several rotational perturbations in the upper A ′ 5 Δ are noted. Possible candidates for the perturbing states are discussed.

The A7Π-X7Σ+ (0, 0) band of MnH at 568 nm has been recorded by laser fluorescence excitation spectroscopy. The original rotational analysis of Nevin [Proc. R. Irish Acad.48A, 1–45 (1942); 50A, 123–137 (1945)] has been extended with some corrections at low J. Systematic internal hyperfine perturbations in the X7Σ+ state, caused by the ΔN = 0, ΔJ = ±1 matrix elements of the 55Mn hyperfine term in the Hamiltonian, have been observed in all seven electron spin components over the entire range of N″ studied. These perturbations destroy the “goodness” of J″ as a quantum number, giving rise to hyperfine-induced ΔJ = ±2 rotational branches and to observable energy shifts of the most severely affected levels. The A7Π state, with A = 40.5 cm−1 and B = 6.35 cm−1, evolves rapidly from Hund's case (a) to case (b) coupling, which produces anomalous branch patterns at low J. A total of 156 rotational branches have been identified and fitted by least squares to an effective Hamiltonian, providing precise values for the rotational and fine structure constants. Values of the principal constants determined in the fit are (1σ errors in units of the last digit are listed in parentheses): The fine structures of the A7Π and X7Σ+ states confirm the assignment of the A ← X transition as Mn 4pπ ← 4sσ in the presence of a spectator, nonbonding Mn 3d5 (6S) open core.

The low-J structure of the B4Π-X4Σ− (1, 0) band of VO has been investigated at sub-Doppler resolution by intracavity laser-induced fluorescence spectroscopy. Values of the principal hyperfine parameters a and (b + c) for the B4Π state have been deduced from the lines of the 4 Π 5 2 -X and 4 Π 3 2 -X subbands: a = 0.00964 cm−1 and (b + c) = −0.01295 cm−1. Estimates of the dipolar parameter c show that it is small compared to b, so that the Fermi contact parameter b is sizeable and negative; this allows the electron configuration of the B4Π state to be assigned as (3dδ)2(3dπ)1(4sσ)1. The very great sensitivity of intracavity laser spectroscopy in the visible region is emphasized.

Sub-Doppler spectra of the A 6 Σ + -X 6 Σ + electronic transition of gaseous MnO are reported. It is found that both electronic states contain “internal hyperfine perturbations” where the hyperfine levels of different electron spin components interact through matrix elements of the hyperfine Hamiltonian off-diagonal in the quantum number J. The ν=0 level of the A 6 Σ + state is severely perturbed by another electronic state so that its internal hyperfine perturbations are overwhelmed; in consequence the A-X (0, 0) band shows a characteristic pattern of line doublings, resulting from the internal perturbations of the X 6 Σ + state alone. The A 6 Σ + , ν=1 level is less perturbed than the ν=0 level, so that the A-X (1, 0) band shows the effects of the internal perturbations in both electronic states. By chance, these occur at corresponding places in the level structure, between the F 3 and F 4 electron spin components. Since the correspondence is fairly exact, the effects of the two internal perturbations partly cancel. No extra lines are induced, even though the hyperfine patterns are highly anomalous. Detailed calculations of the hyperfine line strengths confirm that the extra lines should be vanishingly weak in this case. The criteria for the appearance of internal hyperfine perturbations, and their value, are discussed.

The (6, 1) and (13, 6) vibrational bands of the A2Πu-X2Σg+ electronic transition of N2+ have been recorded in absorption, at Doppler-limited resolution, by the technique of velocity modulation. The rotational fine structure was fitted by least squares to standard expressions. The rotational and translational temperatures have been measured and indicate an equilibrium between translational and rotational motion of N2+ in the HeN2 plasma.

A small rotational perturbation has been found in the 5Δi ground state of FeO. This occurs in the v = 2 level of the Ω = 2 substate, and only one Λ-boudling component of one rotational level is affected. The perturbing state, which lies about 2100 cm−1 above the lowest spin-orbit level of the molecule, is orbitally nondegenerate, and is likely to belong to the 7Σ+ state arising from the configuration (4sσ)1(3dδ)2(3dπ)2(3dσ)1. The new state, a7Σ+, is possibly responsible for anomalies in the intensity pattern of the FeO− photodetachment spectrum of Engelking and Lineberger [J. Chem. Phys., 66, 5054–5058 (1977)]. Improved vibrational and rotational constants are presented for the ground state, combining new Fourier transform measurements of the Λ-doubling in X5Δ1 with the recent microwave data of Endo et al. [Astrophys. J., 278, L131–132 (1984)].

The C4Σ−-X4Σ− (0, 0) band of NbO, whose R heads lie near 4690 A, has been analyzed from high-resolution grating spectra. Both 4Σ− states have second-order spin-orbit splittings, 4 λ = E( 4 Σ 3 2 )-E( 4 Σ 1 2 ) , of about 60 cm−1, so that they are in good case (a) coupling at low J, although the appearance of the band had been interpreted by previous workers as 4Σ(b)-4Σ(b). the 93Nb nuclear hyperfine structure ( I= 9 2 ) is very impressive in the R1, P1, R4, and P4 branches, with the 10 hyperfine components of a rotational line spread over more than 0.9 cm−1 in the F1 branches. The hyperfine linewidths vary considerably with J, in the manner expected for the rapid spin uncoupling from case (aβ) to (bβJ) which occurs in a high-multiplicity state. The very great hyperfine linewidths arise because the sign of the Fermi contact parameter b (the coefficient of I·S in the magnetic hyperfine Hamiltonian) is different in the two 4Σ states, so that the observed patterns are the sums of the splittings in the two electronic states.

The rotational structure of the 000-000 band of the 2490-A system of 15 NO 2 ( 2 2 B 2 - X 2 A 1 ) has been analyzed from high dispersion grating spectrograph plates. The band is found to be slightly predissociated, exactly as in the 14 NO 2 isotope, which suggests that it might be usable for laser separation of the isotopes of nitrogen; tables of the wavenumbers of the lines are given. The upper-state molecular constants are close to the values calculated by the isotope relations from those of 14 NO 2 .

The A4Π-X4Σ− electronic transition of VO in the near infrared was recorded at Doppler-limited resolution by Fourier transform spectroscopy, and rotational analyses performed for the (0, 0) band at 1.05 μm and the (0, 1) band at 1.18 μm. The A4Π state is found to have comparatively small spin-orbit coupling (A = 35.19 cm−1) so that is is almost completely uncoupled to case (b) at the highest N values observed, near N = 90. The hyperfine structure due to the 51V nucleus ( I = 7 2 ) is prominent in the 4 Π 5 2 -X 4 Σ − subband, and in many of the spin satellite branches; at high N values, where only main branches (ΔN = ΔJ) occur, the lines are sharp, indicating that the hyperfine b parameter (the coefficient of I·S in the magnetic hyperfine Hamiltonian) is the same in the A4Π and X4Σ− states. The electron configuration of the A4Π state is therefore (4sσ)1(3dδ)1(4pπ)1.

The first high-resolution (0.0024 cm −1 ) spectrum of the 2 0 1 band of COF 2 at 963 cm −1 is reported. Nearly 4000 rotational transitions have been observed and assigned in the band between 936 and 990 cm −1 . The line positions are estimated to be accurate to 0.0001 cm −1 (relative). The spectrum was calibrated using two adjacent bands of OCS. Approximately 1560 infrared transitions have been fitted simultaneously with the previously reported microwave data. The wave numbers of far infrared laser lines recently observed by Tobin and by Temps and Wagner have been calculated from the 2 1 energy levels, with estimated uncertainties of between 10 −5 and 10 −6 cm −1 .

Two new singlet electronic transitions, consisting of sequences near 8400 and 9200 A, have been recorded by Fourier transform emission spectroscopy, where the source was a microwave discharge in flowing VOCl3 vapor mixed with helium. Rotational analysis shows that these are probably 1Σ+-1Σ+ and 1Δ-1Δ transitions, and arguments are presented indicating that the carrier is VO+, for which no previous spectroscopic observations have been reported. The lower electronic states of both transitions show highly anomalous centrifugal distortion effects, presumably as a result of rotational perturbations: The perturbing electronic state may be the 3Δ state which arises from the same configuration as the lower 1Δ state.

Large numbers of ‘hot’ bands in the first electronic absorption systems of 12CS2 and 13CS2 have been analyzed from plates taken at high dispersion, and accurate rotational constants have been obtained for the overtones of the ground state bending vibration up to v2 = 6 and l = 3 for 12CS2 and v2 = 4, l = 2 for 13CS2. The energy differences between the various levels with the same l value have been determined to an accuracy of about ±0.006 cm−1, but (because of the parallel polarization of the electronic transition) the absolute energies of levels with l > 0 cannot be obtained.

Extensive laser excitation spectra and rotationally resolved laser-induced fluorescence spectra have been recorded for the “orange system” of gaseous FeO in the wavelength regions 5790–6140 and 5580–5640 A. Detailed rotational analyses have been performed for about 20 Ω ′ substates lying between 16 350 and 18 550 cm−1. These are found to comprise a very severely perturbed 5Δi excited electronic state with a bond length of about 1.69 A (which is responsible for the parallel polarization of the electronic transition from the 5Δi ground electronic state) and a large number of “extra” Ω substates with B′ values ranging from 0.38 to 0.50 cm−1, which almost certainly belong to high vibrational levels of lower-lying electronic states. Evidence about the natures of the “extra” states is confusing, however, with the 54FeO56FeO isotope shifts apparently being in conflict with the patterns of vibrationally resolved laser-induced fluorescence. Every single Ω substate that has been analyzed shows rotational perturbations of varying severity. The density and magnitude of the rotational perturbations are quite exceptional for a diatomic molecule, and result in a new type of totally chaotic diatomic spectrum. There is a remarkable similarity to the visible spectrum of NO2: in NO2 the complications arise from the high density of perturbing ground state vibrational levels; in FeO there is a correspondingly high density of perturbing electronic states at lower energy. The great complexity of the FeO spectrum arises because the states are in an awkward intermediate spin-coupling case which still resembles Hund's case (a) but shows strong tendencies toward Hund's case (c) coupling.

The emission spectrum of VO in the near-infrared has been studied by high-resolution Fourier transform spectroscopy. Rotational analyses of several new electronic transitions have been performed, which more than double the number of known states of VO. Specifically, two quartet transitions, D 4 Δ- A ′ 4 Φ near 11 800 cm −1 and D 4 Δ- A 4 Π near 9600 cm −1 , have given accurate data (including the absolute energies relative to X 4 Σ − ) for the states σδσ ∗ D 4 Δ and σδπ A ′ 4 Φ, and complete the characterization of the predicted low-lying quartet states of VO; these transitions also appear to be the first examples of their types to have been analyzed in detail. A new 2 Δ- 2 Φ transition near 9600 cm −1 (overlapping D 4 Δ- A 4 Π) has been assigned to one of the possible σδσ ∗ 2 Δ- σδπ 2 Φ transitions, and a complex group of 2 Π- 2 Δ transitions in the 7000 to 8000-cm −1 region has given data for the low-lying σ 2 δ 2 Δ state and for parts of two 2 Π states. Perturbations are widespread throughout the spectra. Using arguments based on the vanadium nuclear hyperfine structure, the spin-orbit coupling, and the bond lengths, it has been possible to assign electron configurations for all the observed electronic states; about half the predicted low-lying states of VO are now known.

The method of laser electric resonance has been applied to the observation of the ν2 fundamental band of the HCO radical using 12C16O2 and 12C18O2 infrared laser lines. Forty-three resonances have been assigned to six low-J vibration-rotation transitions, qQ1(1), qP1(2), qR1(1), qQ2(2), qP2(3), and qR2(2), each having spin-rotation splitting and K-doubling. The resonances have been analyzed using the detailed theory of the Stark effect in doublet electronic states of asymmetric top molecules. Using previous microwave values for the parameters in the ground state of HCO, the following constants have been determined: ν 0 +4(Δ′ K − Δ″ K ) = 1080.8392(16) cm −1 , A′ − A″ −5(Δ′ K − Δ″ K ) = 2.15146(42) cm −1 , B′ + C′= 2.89259(60) cm −1 , B′ − C′= 0.10572(105) cm −1 , ϵ″ aa = 0.3885(30) cm −1 , ϵ′ aa = 0.4701(36) cm −1 , η″ aaaa = −0.0016(10) cm −1 , η′ aaaa = −0.0018(13) cm −1 , μ″ a = 1.3626(39) debye , μ′ a = 1.3474(48) debye . The variation of the molecular constants with excitation of the ν2 vibration and the relative merits of electric and magnetic resonance methods are discussed.

The emission spectrum of CrO has been investigated by Fourier transform spectroscopy in the near infrared. New weak electronic bands have been found in the 6000- to 10 000-cm−1 region, the strongest of which, near 8000 cm−1, is shown to be the (0,0) band of a 5Σ-5Π transition where the 5Π lower state is the ground state. Fifty branches have been assigned in this band, which have permitted the first detailed characterizations of quintet electronic states in the gas phase. Accurate values have now been obtained for the spin-orbit coupling and Λ-doubling intervals in the ground state (which could only be estimated in the previous laser-induced fluorescence work in the visible region by Hocking et al. [Canad. J. Phys.58, 516–533 (1980)]). The relative branch intensities are not consistent with those calculated for a pure 5Σ-5Π(a) transition, and indicate considerable spin-orbit contamination such that there are interference effects between two or more competing transition moments. It is not known whether the 5Σ excited state is 5Σ+ or 5Σ−.

Laser-induced fluorescence spectra of gaseous FeO have proved that the bands whose P and R branches have been analyzed rotationally by Harris and Barrow are Ω ′ = 4 − Ω ″ = 4 transitions. Since the lower state of these bands has the same vibrational frequency as that observed for matrix-isolated FeO at 14 K by Green, Reedy and Kay, the ground state of FeO must contain an Ω = 4 spin-orbit component. Consideration of the low-lying m.o.'s of the transition metal oxides shows that (4sσ) 1 (3dδ) 3 (3dπ) 2 5 Δ i is the only reasonable assignment for the ground state of FeO consistent with this result; the other possible assignments, (4sσ) 2 (3dδ) 2 (3dπ) 2 5 Σ + and (4sσ) 1 (3dδ) 2 (3dπ) 2 (3dσ) 1 7 Σ + , do not give rise to Ω = 4 components.

A detailed vibrational analysis of the 5515 A absorption of s -tetrazine (tetra-azabenzene) vapor is attempted. More than 80% of the total intensity of the system can be accounted for by band assignments to a single allowed electronic transition, 1 B 3 u - 1 A g . The only progression-forming vibration in absorption in ν 6 a . Three progressions in ν ′ 6 a account for all intense cold bands. Isotopic shifts of bands lead to convincing assignments of the two excited-state vibrations upon which ν 6 a progressions are based, namely ν 6 b and ν 8 a . It is shown that in all probability the antisymmetric mode ν 6 b is prominent because of vibrational (Fermi) resonance rather than because of vibronic interactions.