Low-lying bending vibronic bands of the MgNC A 2Pi-X 2Sigma+ transition

We have generated MgNC in supersonic free jet expansions and measured the laser induced fluorescence excitation spectra of the Mg-N-C bending vibronic bands of the A 2Pi-X 2Sigma+ transition. In addition to the two vibronic bands, 2(0) (1), kappa 2Sigma(+)- and 2(0) (2), kappa 2Pi-2Sigma+, reported previously, the 2(0) (2), mu 2Pi1/2-(2)Sigma+ vibronic subband was found just above the 2(0) (1) band. The most remarkable feature of this subband is unexpected rotational structure of the A (020) mu 2Pi1/2 level, showing the splitting of the e and f sublevels. On the basis of the fact that the A (020) mu 2Pi1/2 level lies very close to the A (010) kappa2Sigma+ level, the ef splitting is ascribed to P-type doubling which is induced by Coriolis interaction between these two bending vibronic levels. Introducing the Coriolis coupling terms arising from the G-uncoupling operator, -J+/-G22-/+, and the spin-Coriolis interaction, S+/-G22-/+, into the rotational Hamiltonian, this unexpected rotational structure has been analyzed. This P-type doubling would be one of the rare examples exhibiting the Coriolis interaction between two bending vibronic levels with Deltav2=+/-1 and Deltal=-/+1. Through the molecular constants of the A (010) kappa 2Sigma+, (020) kappa 2Pi, and mu 2Pi1/2 levels, the Renner-Teller vibronic structure of the nu2 bending mode in the A 2Pi state has been characterized. The observed vibronic bands analyzed in this study show some anomalies in the band intensities. Based on the information of the nu2 bending vibronic structure derived from the present analyses, we discuss the intensity anomalies.     

Experimental and theoretical investigation of the dispersed fluorescence spectroscopy of HC4S

A high-resolution single vibronic level emission study from the A (2)Pi(32) state of the HC(4)S radical is reported. Ground state density functional theory frequencies have been used to assign ground state vibronic levels involving three stretching modes nu(2), nu(3), and nu(5) in the region of 0-3250 cm(-1), while the frequency of nu(4) remains speculative. Tentative assignments are given for the complicated structures arising from Renner-Teller and spin-orbit interactions within the bending energy levels. From analysis of the dispersed emission spectra, Fermi resonances involving pairs of bands have been identified in the A (2)Pi(32)<--X (2)Pi(32) laser induced fluorescence spectrum.     

Spectroscopy, dissociation dynamics, and potential energy surfaces for CN(A)-Ar

The A (2)Pi-X (2)Sigma(+) band system of CN-Ar has been examined using fluorescence depletion and action spectroscopy techniques. Eight vibronic bands of the complex were observed in association with the monomer 3-0 transition. Pump-probe measurements were used to characterize CN(A (2)Pi(32),nu=3) fragments from direct photodissociation of CN(A (2)Pi,nu=3)-Ar and CN(X (2)Sigma(+),nu=7) fragments from CN(A (2)Pi,nu=3)-Ar predissociation. The latter showed a marked preference for population of positive parity diatomic rotational levels. Bound state calculations were used to assign the A-X bands and to obtain fitted potential energy surfaces for the A state. The average potential obtained from fitting had a well depth of D(e)=137.8 cm(-1). High-level ab initio calculations were used to obtain equilibrium Jacobi coordinates of theta(e)=94 degrees and R(e)=7.25 bohr. The near-symmetric character of the fitted potential energy surface was consistent with the symmetry preference observed in the predissociation dynamics.     

Parallel and coupled perpendicular transitions of RbCs 640 nm system: mass-resolved resonance enhanced two-photon ionization in a cold molecular beam

We have investigated the RbCs 640 nm system by mass-resolved resonance enhanced two-photon ionization in a cold molecular beam. Very complex vibronic structures were observed between 15420 and 15990 cm (-1). The parallel transitions of 2 (3)Pi 0 v' = 4-20 <-- X (1)Sigma (+) v' = 0 were identified by rotationally resolved spectra. Molecular constants and a Rydberg-Klein-Rees potential energy curve of the 2 (3)Pi 0 state were determined. The regular vibrational spacing of the parallel transition indicated that the 2 (3)Pi 0 state is not significantly perturbed by nearby excited electronic states. The complexity of the observed vibronic structures has been attributed to the coupled perpendicular transitions of 2 (1)Pi, 2 (3)Pi 1, and 3 (3)Sigma 1 (+) <-- X (1)Sigma (+) v' = 0. For the perpendicular bands observed in the lower-energy spectral region between 15420 and 15630 cm (-1) where the onsets of the 2 (3)Pi 1 and 3 (3)Sigma 1 (+) <-- X (1)Sigma (+) transitions are located, the upper electronic states and the vibrational quantum numbers were assigned. Perturbations of 2 (3)Pi 1-3 (3)Sigma 1 (+) and 2 (1)Pi-3 (3)Sigma 1 (+) have been identified by the observed level shifts.     

Laser induced fluorescence spectroscopy of the C3N radical

Electronic spectra of the C3N radical have been observed for the first time in the near ultraviolet wavelength region by laser induced fluorescence (LIF) spectroscopy. Seventeen vibronic bands of the B 2Pii-X 2Sigma+ electronic transition system of C3N were identified in LIF spectra of products in a discharge of HC3N. The origin of the B 2Pii state was determined to be 27,929.985(1) cm(-1) from rovibrational analyses. It was found that observations of two types of 2Sigma vibronic levels, which have 2Sigma+ and 2Sigma+/- symmetries originated from excitations of the nu4 trans-bending mode (omega4=369.1(20) cm(-1)) with a large Renner-Teller (RT) interaction (epsilon4=-0.1549(50)), and the nu5 cis-bending mode (omega5=163.24(84) cm(-1)) with a small Renner-Teller interaction (epsilon5=-0.0503(68)), respectively. Vibronic levels, with excitations of the C-C stretching (omega3=869.7 cm(-1)) mode, were also identified. The spin-orbit interaction constant was determined to be Aso=-36.7(50) cm(-1) from the RT analysis. In dispersed fluorescence spectra from B 2Pii, vibrational structures of the low-lying electronically excited A 2Pii state were clearly observed with a strong progression due to the nu3' mode, together with those of the X 2Sigma+ state with weak intensities. The origin of A 2Pii, T0=1844(3) cm(-1), and the vibrational frequencies, omega3'=883(3) cm(-1) and omega5'=121(3) cm(-1) for A 2Pii, and omega3"=1054(3) cm(-1), omega4"=405(3) cm(-1), and omega5"=131(3) cm(-1) for X 2Sigma+, were determined. Time profiles of fluorescence from B 2Pii have short (50-200 ns) and long (>1 micros) decay components with quantum beats, indicating that there is a competition between radiative decay and the nonradiative internal conversion to vibrationally highly excited A 2Pii and X 2Sigma+.     

Renner-Teller effect in C2H2+(X2Pi u) studied by rotationally resolved zero kinetic energy photoelectron spectroscopy

The Renner-Teller effect in C(2)H(2)(+)(X(2)Pi(u)) has been studied by using zero kinetic energy (ZEKE) photoelectron spectroscopy and coherent extreme ultraviolet (XUV) radiation. The rotationally resolved vibronic spectra have been recorded for energies up to 2000 cm(-1) above the ground vibrational state. The C triple bond C symmetric stretching (upsilon(2)), the CCH trans bending (upsilon(4)), and the CCH cis bending (upsilon(5)) vibrational excitations have been observed. The assigned vibronic bands are 4(1)(1)(kappa(2)Sigma(u)(+))(hot band), 4(1)(0)(mu/kappa(2)Sigma (u)(-/+)), 5(1)(0)(mu/kappa(2)Sigma (g)(+/-)), and 4(2)(0)(mu(2)Pi(u)), 4(2)(0)(kappa(2)Pi(u)), 4(1)(0)5(1)(0) (mu(2)Pi(g)), 0(0)(0)(X(2)Pi(u)), and 2(1)(0)(X(2)Pi(u)). The Renner-Teller parameters, the harmonic frequencies, the spin-orbit coupling constants, and the rotational constants for the corresponding vibronic bands have been determined by fitting the spectra with energy eigenvalues from the Hamiltonian that considers simultaneously Renner-Teller coupling, vibrational energies, rotational energies, and spin-orbit coupling interaction.     

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

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

Is the HCCS radical linear in the excited state?

The A (2)Pi-X (2)Pi 415 nm band system of the linear HCCS radical has been known since 1978, but the vibronic structure in this complex spectrum, which has both spin-orbit and Renner-Teller complications, has never been satisfactorily assigned, despite serious experimental and theoretical efforts. In a further attempt to understand the spectrum, we have studied the laser-induced fluorescence spectra of jet-cooled HCCS and DCCS, produced from thiophene precursors using the discharge jet technique. The 0(0) (0) bands of HCCS and DCCS have been rotationally analyzed, providing precise ground and excited state spin-orbit splittings. The energy levels of the v(')=0 (2)Pi(3/2) component of DCCS are found to be perturbed by a very low-lying (2)Sigma vibronic level, indicating that the HCC bending mode Renner-Teller effect is much larger than predicted by ab initio calculations with a linear excited state geometry. With this observation, the vibronic bands in the spectra of both isotopomers have been consistently assigned for the first time. Model calculations show that the large Renner-Teller effect and substantially different HCCS and DCCS excited state zero-point spin-orbit splittings can be explained with the assumption of a quasilinear excited state geometry.     

Laser-induced fluorescence spectroscopy of NC3S

In a discharged supersonic jet of acetonitrile and carbon disulfide, we have for the first time observed an electronic transition of the NC(3)S radical using laser-induced fluorescence (LIF) spectroscopy. A progression originating from the C-S stretching mode of the upper electronic state appears in the excitation spectrum. Each band of the progression has a polyad structure due to anharmonic resonances with even overtones of bending modes. Rotationally resolved spectra have been observed by high-resolution laser scans, and the electronic transition is assigned to A 2Pii-X 2Pii. For the vibronic origin band, the position and the effective rotational constant of the upper level have been determined to be 21 553.874(1) and 0.046 689(4) cm(-1), respectively. The dispersed fluorescence spectrum from the zero vibrational level of A 2Pi3/2 has also been observed; its vibrational structure is similar to that of the LIF excitation spectrum, showing a prominent C-S stretching progression with polyad structures. The vibrational frequencies of the C-S stretching mode in the ground and excited electronic states are determined to be 550 and 520 cm(-1), respectively. Fluorescence decay profiles have been measured for several vibronic levels of the A state.     

In search of the germanium halomethylidyne (Ge=C-X; X=F,Cl,Br) free radicals: Ab initio studies of their spectroscopic signatures

A variety of ab initio methods have been used to calculate the X (2)Pi and A (2)Sigma(+) state spectroscopic parameters of the GeCX (X=F,Cl,Br) free radicals. The theoretical methods and basis sets were tested on GeCH, for which extensive experimental data are available, and found to give predictions sufficiently reliable to guide experimental searches for spectra. In all cases, the linear Ge=C-X species was found to be the global minimum on the potential energy surface, with the bent X-Ge=C ((2)A(')) isomer as a local minimum much higher (62-36 kcal/mol) in energy. In both the ground and excited states, the GeC moiety is very similar to that of GeCH, with a double bond in the lower state and a triple bond in the excited state, indicating that halogenation does not radically perturb the energetics or structure of germanium methylidyne. Ground state GeCX radicals have suitable rotational constants for microwave studies, although they suffer from only modest dipole moments. Matrix infrared experiments are most likely to detect the nu(1) fundamentals in the 1450-1100 cm(-1) region or the nu(3) fundamentals at the transition between the mid- and far-infrared regions. We have used the ab initio values for the Renner-Teller parameter, the average bending frequency, and the spin-orbit coupling constant to calculate the ground state energy levels, which will be helpful in the interpretation of A-X single vibronic level emission spectra, if they can be observed. The electronic absorption spectra of the (2)Pi(32) spin component of the 0(0) (0) bands of all three radicals have been calculated assuming typical jet-expansion conditions and should be useful in future laser-induced fluorescence, resonance enhanced multiphoton ionization, or cavity ringdown searches for the electronic band systems.     

A vacuum ultraviolet pulsed field ionization-photoelectron study of cyanogen cation in the energy range of 13.2-15.9 eV

The vacuum ultraviolet pulsed field ionization-photoelectron and photoionization efficiency spectra of NCCN have been measured in the energy region of 13.25-17.75 eV. The analyses of these spectra have provided accurate ionization energy (IE) values of 13.371+/-0.001, 14.529+/-0.001, 14.770+/-0.001, and 15.516+/-0.001 eV for the formation of NCCN(+) in the X(2)Pi(g), A(2)Sigma(g) (+), B(2)Sigma(u) (+), and C(2)Pi(u) states, respectively. The ionization energy [NCCN(+)(B(2)Sigma(u) (+))] value determined here indicates that the origin of the NCCN(+)(B(2)Sigma(u) (+)) state lies lower in energy by 25 meV than previously reported. A set of spectroscopic parameters for NCCN(+)(X(2)Pi(g)) has been calculated using high level ab initio calculations. The experimental spectra are found to consist of ionizing transitions populating the vibronic levels of NCCN(+), which consist of pure vibronic progressions, combination modes involving the symmetric CN stretch, the CC stretch, and even quanta of the antisymmetric CN stretch, and bending vibrations. These bands are identified with the guidance of the present ab initio calculations.     

Fluorescence spectrum of the benzyl radical in methylcyclohexane at 4.2 K

The fluorescence spectrum of the benzyl radical (BR) is characterized by participation of many totally symmetric vibrational modes with less intensity activeness. To make this point more clear, the fluorescence spectra of the benzyl radical and its deuterated compounds in methylcyclohexane (MCH) have been observed at 4.2 K and detailed vibrational analysis has been carried out. Two non-totally symmetric modes such as nu6b and nu8b play a role of Herzberg-Teller type vibronic coupling together with one totally symmetric mode nu6a. There are only a few progression bands with very weak intensity. These are nu12, nu1 and nu(phi-CH2) vibrational modes.     

Fluorescence and metastability of N2O2+: theory and experiment

State-selective mass spectrometry has revealed one conclusive and another probable metastable state of the N2O2+ dication, assigned respectively as 1 3Pi at 38.5 eV and 2 3Pi at 42.5 eV. Photon coincidence experiments confirm that dissociation of 1 3Pi is preceded by a fluorescent transition to X 3Sigma- and also indicate that an identical mechanism occurs for 2 3Pi. Highly correlated MRCI calculations are performed at a range of N2O2+ geometries, from which both N-N and N-O bond stretching curves are generated. Substantial barriers along both coordinates are observed for 1 3Pi and 2 3Pi, although the increasing density of states at higher energy may allow spin-orbit or vibronic predissociation for 2 3Pi. Fragment emissions derived from N2O+ and N2O2+ are analyzed with the aid of glass filters, from which NO (X 2Pi<--A 2Sigma+) and vibrationally excited N2+ (X 2Sigmag+<--B 2Sigmau+) transitions are deduced.     

(2+1) resonance-enhanced ionization spectroscopy of a state-selected beam of OH radicals

A state-selected beam of hydroxyl radicals is generated using a pulsed discharge source and hexapole field. The OH radicals are characterized by resonance-enhanced multiphoton ionization (REMPI) spectroscopy via the nested D 2Sigma- and 3 2Sigma- Rydberg states. Simplified spectra are observed from the selected |MJ|=3/2 component of the upper Lambda-doublet level of the lowest rotational state (J=32) in ground (v"=0) and excited (v"=1-3) vibrational levels of the OH X 2Pi3/2 state. Two-photon transitions are observed to the D 2Sigma-(v'=0-3) and 3 2Sigma-(v'=0,1) vibronic levels, extending previous studies to higher vibrational levels of the Rydberg states. Spectroscopic constants are derived for the Rydberg states and compared with prior experimental studies. Complementary first-principle theoretical studies of the properties of the D 2Sigma- and 3 2Sigma- Rydberg states [see M. P. J. van der Loo and G. C. Groenenboom, J. Chem. Phys. 123, 074310 (2005), following paper] are used to interpret the experimental findings and examine the utility of the (2+1) REMPI scheme for sensitive detection of OH radicals.     

Persistence rewarded: successful observation of the B 2Sigmau+-X 2Pig electronic transition of jet-cooled BS2

The B-X electronic transition of jet-cooled BS2 has been observed using laser-induced fluorescence techniques. The boron disulfide radical was produced in a pulsed electric discharge jet using a mixture of BCl3 and CS2 in high-pressure argon as the precursor. The spectrum consists of a strong 0(0)(0) band with the 2Sigma-2Pi(3/2) component at 24,393.2 cm(-1) and short progressions in the symmetric stretching (nu1' = 506.7 cm(-1)) and bending (nu2' = 303.2 cm(-1)) modes. A rotational analysis of both spin-orbit components of the 0(0)(0) band gave an upper state B value of 0.0932779(19) cm(-1) and a ground-state spin-orbit coupling constant of A = -405.163(4) cm(-1). The ground-state bond length of 1.66492 angstroms increases to 1.6812(1) angstroms on sigmau --> pig electronic excitation. The B-X data have been used to further refine the Renner-Teller analysis, which is in good agreement with our previous work [J. Chem. Phys. 119, 2047 (2003)].     

The 39K(2) 2(3)Sigma(g)+ state: observation and analysis

The (39)K(2) 2 (3)Sigma(g) (+) state has been observed by perturbation-facilitated infrared-infrared double resonance spectroscopy and two-photon excitation. Resolved fluorescence spectra into the a (3)Sigma(u) (+) state have been recorded. The observed vibrational levels have been assigned as the v=23-25, 27, 28, 31-33, 38-45, 47, and 53 levels by comparing the observed and calculated spectra of the 2 (3)Sigma(g) (+)-->a (3)Sigma(u) (+) transitions. Molecular constants have been obtained using a global fitting procedure with a comprehensive set of experimental data. Fine and hyperfine splittings have been resolved in the excitation spectra. Perturbations between the 2 (3)Sigma(g) (+) and 2 (3)Pi(g) states were observed. The hyperfine patterns of the 2 (3)Sigma(g) (+) levels are strongly affected by the perturbation. The perturbation-free and weakly perturbed levels follow the case b(betaS) coupling scheme, while the perturbed levels follow case b(beta J) coupling. A Fermi contact constant, b(F)=65+/-10 MHz, has been obtained. Intensity anomalies of rotational lines appeared both in the 2 (3)Sigma(g) (+) approximately 2 (3)Pi(g)<--b (3)Pi(u) excitation spectra and in the 2 (3)Sigma(g) (+) approximately 2 (3)Pi(g)-->a (3)Sigma(u) (+) resolved fluorescence spectra. These intensity anomalies can be explained in terms of a quantum-mechanical interference effect.     

The molecular structure and a Renner-Teller analysis of the ground and first excited electronic states of the jet-cooled CS2 + molecular ion

The A 2Pi(u) - X 2Pi(g) electronic band system of the jet-cooled CS2 + ion has been studied by laser-induced fluorescence and wavelength-resolved emission techniques. The ions were produced in a pulsed electric discharge jet using a precursor mixture of carbon disulfide vapor in high-pressure argon. Rotational analysis of the high-resolution spectrum of the 2Pi32 component of the 0(0) 0 band gave linear-molecule molecular structures of r0" = 1.5554(10) A and r0' = 1.6172(12) A. Renner-Teller analyses of the vibronic structure in the spectra showed that the ground-state spin-orbit splitting (A = -447.0 cm(-1)) is much larger than that of the excited state (A = -177.5 cm(-1)), but that the Renner-Teller parameters are of similar magnitude and that a strong nu1 - 2nu2 Fermi resonance occurs in both states. Previous analyses of the vibronic structure in the ground and excited states of the ion from pulsed field-ionization-photoelectron data are shown to be substantially correct.     

Infrared action spectroscopy of the OD stretch fundamental and overtone transitions of the DOOO radical

The DOOO radical has been produced by three-body association between OD and O2 in a supersonic free-jet expansion and investigated using action spectroscopy, an IR-UV double-resonance technique. Partially rotationally structured bands observed at 2635.06 and 5182.42 cm(-1) are assigned to the OD stretch fundamental (nu(OD)) and overtone (2nu(OD)), respectively, of the trans-DOOO radical. Unstructured bands observed in both spectral regions are assigned to cis-DOOO. Nascent OD X(2)Pi product state distributions following vibrational predissociation appear to be nearly statistical with respect to the degree of rotational excitation, but display a marked propensity for Pi(A') Lambda-doublets, which is interpreted as a signature of a planar dissociation. The energetically highest open OD X(2)Pi product channel implies an upper limit dissociation energy D0 < or = 1856 cm(-1) or 5.31 kcal mol(-1). This value allows refinement of the upper limit D0 of the atmospherically important HOOO isotopomer, suggesting that it is marginally less stable than previously thought.     

Gas phase electronic spectrum of linear AlCCH

The electronic spectrum of the aluminium containing species AlCCH has been detected in the gas phase in the region 315-355 nm. The experiment used a mass selective resonant two-color two-photon ionization technique coupled to a laser ablation source. Structures of the AlCCH isomers have been optimized using density functional theory (DFT) and the excitation energies to the low-lying electronic excited states calculated. Based on the analysis of the observed rotational structure and the theoretical data, the spectrum is assigned to the A (1)Pi<-- X (1)Sigma(+) electronic transition of linear AlCCH. The vibronic band system is complicated by the Renner-Teller effect in the excited state. The assignment yields nu(4)' = 516.4 cm(-1) for the stretching mode in the ground X (1)Sigma(+) state and nu(4)' = 654.5 cm(-1) for A (1)Pi excited state. Molecular constants determined from the rotational analysis are B(0)' = 0.16487(14), B(0)' = 0.17845(13) and T(0) = 28 755.04 cm(-1). The experimental and theoretical data indicate a shorter Al-C bond in the A (1)Pi excited than the X (1)Sigma(+) ground state.