B1(1)Π state of KCs: high-resolution spectroscopy and description of low-lying energy levels

The diode laser induced B(1)(1)Π → X(1)Σ(+) fluorescence spectra of the KCs molecule were recorded by Fourier-transform spectrometer with resolution of 0.03 cm(-1). Buffer gas Ar was used to facilitate appearance of rotation relaxation lines in the spectra, thus enlarging the B(1)(1)Π dataset, allowing one to determine the Λ-splitting constants and to reveal numerous local perturbations. A dataset was systematically obtained for B(1)(1)Π vibrational levels ν(') ∈ [0; 5] nonuniformly covering rotational levels with J(') ∈ [7; 233]. For ν(') ∈ [0; 3], the less-perturbed data of f-components were included in the fit. A pointwise potential energy curve (PEC) based on the inverted perturbation approach, as well as the Dunham coefficients, was obtained and compared with ab initio calculations; in particular, the energy of the PEC's minimum T(e) = 14,044.918(6) cm(-1) was determined. Both approaches allowed us to reproduce the vast majority of data used in the fit with accuracy of 0.02 cm(-1). Tentative molecular constants for several vibrational levels of the near lying C(3)(1)Σ(+) state were estimated.     

The ν2 bending vibrational structure of the X̃2Σ+ state of MgNC

We have generated MgNC in supersonic free jet expansions and observed the laser induced fluorescence (LIF) of the A?(2)Π-X?(2)Σ(+) transition. We measured the LIF dispersed spectra from the single vibronic levels of the A?(2)Π electronic state of MgNC, following excitation of each ν(2) bending vibronic band observed, i.e., the κ series of the (0,v(2)('),0)-(0,0,0), v(2)(') = 0, 1, 2, 4, and 6 vibronic bands. In the vibrational structure in the dispersed fluorescence spectra measured, the long progression of the ν(2) bending mode in the X?(2)Σ(+) state is identified, e.g., up to v(2)(')=14 in the (0,6,0)-(0,v(2)('),0) spectrum. This enables us to derive the potential curve of the ν(2) bending mode in the X?(2)Σ(+) state. We used two kinds of models to obtain the potential curve; (I) the customary formula expressed in the polynomial series of the (v(2)(')+(d(2)/2)) term and (II) the internal rotation model. The potential curve derived from model (I) indicates the convergence of the bending vibrational levels at about 800 cm(-1) from the vibrationless level of MgNC, which may correspond to the barrier height of the isomerization reaction, MgNC ? MgCN, in the X?(2)Σ(+) state. Model (II) gives a simple picture for the isomerization reaction pathway with a barrier height of about 630 cm(-1) from the vibrationless level of the more stable species, MgNC. This shows that the v(2)(')=8 bending vibrational level of MgNC is already contaminated by the v(2)(')=2 bending vibrational level of the isomer, MgCN, and implies that the isomerization reaction begins at the v(2) (')=8 level. The bending potential surface and the isomerization reaction pathway, MgNC ? MgCN, in the X?(2)Σ(+) state are discussed by comparing the potential derived in this study with the surface obtained by quantum chemical calculation.     

Electronically excited-state properties and predissociation mechanisms of phosphorus monofluoride: a theoretical study including spin-orbit coupling

The 51 Ω states generated from the 22 Λ - S states of phosphors monofluoride have been investigated using the valence internally contracted multireference configuration interaction method with the Davidson correction and the entirely uncontracted aug-cc-pV5Z basis set. The spin-orbit coupling is computed using the state interaction approach with the Breit-Pauli Hamiltonian. Based on the calculated potential energy curves, the spectroscopic constants of the bound and quasibound Λ - S and Ω states are obtained, and very good agreement with experiment is achieved. Several quasibound states caused by avoided crossings are found. Various curve crossings and avoided crossings are revealed, and with the help of our computed spin-orbit coupling matrix elements, the predissociation mechanisms of the a(1)Δ, b(1)Σ(+), e(3)Π, g(1)Π, and (3)(3)Π states are analyzed. The intricate couplings among different electronic states are investigated. We propose that the avoided crossing between the A(3)Π(0 +) and b(1)Σ(0+) (+) states may be responsible for the fact that the A(3)Π ν' ≥ 12 vibrational levels can not be observed in experiment. The transition properties of the A(3)Π - X(3)Σ(-) transition are studied, and our computed Franck-Condon factors and radiative lifetimes match the experimental results very well.     

Rotational analysis and deperturbation of the A2Π → X2Σ+ and B'2Σ+ → X2Σ+ emission spectra of MgH

Deperturbation analysis of the A(2)Π → X(2)Σ(+) and B(')(2)Σ(+) → X(2)Σ(+) emission spectra of (24)MgH is reported. Spectroscopic data for the v = 0 to 3 levels of the A (2)Π state and the v = 0 to 4 levels of the B'(2)Σ(+) state were fitted together using a single Hamiltonian matrix that includes (2)Π and (2)Σ(+) matrix elements, as well as off-diagonal elements coupling several vibrational levels of the two states. A Dunham-type fit was performed and the resulting Y(l,0) and Y(l,1) coefficients were used to generate Rydberg-Klein-Rees (RKR) potential curves for the A (2)Π and the B'(2)Σ(+) states. Vibrational overlap integrals were computed from the RKR potentials, and the off-diagonal matrix elements coupling the electronic wavefunctions (a(+) and b) were determined. Zero point dissociation energies (D(0)) of the A(2)Π and B'(2)Σ(+) states of (24)MgH were determined to be 12,957.5 ± 0.5 and 10,133.6 ± 0.5 cm(-1), respectively. Using the Y(0,1) coefficients, the equilibrium internuclear distances (r(e)) of the A(2)Π and B'(2)Σ(+) states were determined to be 1.67827(1) ? and 2.59404(4) A?, respectively.     

Communication: A new spectroscopic window on hydroxyl radicals using UV + VUV resonant ionization

A 1 + 1' multiphoton ionization (MPI) detection scheme for OH radicals is presented. The spectroscopic approach combines initial excitation on the well-characterized A(2)Σ(+)-X(2)Π band system with vacuum ultraviolet (VUV) ionization via autoionizing Rydberg states that converge on the OH(+) A(3)Π ion state. Jet-cooled MPI spectra on the (1,0) and (2,0) bands show anomalous rotational line intensities, while initial excitation on the (0,0) band does not lead to detectable OH(+) ions. The onset of ionization with the (1,0) band is attributed to an energetic threshold; the combined UV + VUV photon energies are above the first member of the autoionizing (A(3)Π)nd Rydberg series. Comparison of the OH 1 + 1' MPI signal with that from single photon VUV ionization of NO indicates that the cross section for photoionization from OH A(2)Σ(+), v' = 1 is on the order of 10(-17) cm(2).     

Fourier transform emission spectroscopy of YH and YD: observation of new A1Δ and B1Π electronic states

The emission spectra of YH and YD molecules have been investigated in the 3600-12,000 cm(-1) region using a Fourier transform spectrometer. Molecules were formed in an yttrium hollow cathode lamp operated with a continuous flow of a mixture of Ne and Ar gases, and YH and YD were observed together in the same spectra. A group of bands observed near 1 μm have been identified as 0-0 and 1-1 bands of the A(1)Δ-X(1)Σ(+) and B(1)Π-X(1)Σ(+) transitions of YH and the 0-0 bands of the same two transitions for YD. The A(1)Δ and B(1)Π states of YH are separated by only about 12 cm(-1) and are involved in strong interactions. A perturbation analysis has been performed using the PGOPHER program to fit the two interacting electronic states and spectroscopic parameters for the A(1)Δ and B(1)Π states, including the interaction matrix elements, have been obtained for the first time.     

Photofragmentations, state interactions, and energetics of Rydberg and ion-pair states: two-dimensional resonance enhanced multiphoton ionization of HBr via singlet-, triplet-, Ω = 0 and 2 states

Mass spectra were recorded for one-colour resonance enhanced multiphoton ionization (REMPI) of H(i)Br (i = 79, 81) for the two-photon resonance excitation region 79,040-80,300 cm(-1) to obtain two-dimensional REMPI data. The data were analysed in terms of rotational line positions, intensities, and line-widths. Quantitative analysis of the data relevant to near-resonance interactions between the F(1)Δ(2)(v' = 1) and V(1)Σ(+)(v' = m + 7) states gives interaction strengths, fractional state mixing, and parameters relevant to dissociation of the F state. Qualitative analysis further reveals the nature of state interactions between ion-pair states and the E(1)Σ(+) (v' = 1) and H(1)Σ(+)(v' = 0) Rydberg states in terms of relative strengths and J' dependences. Large variety in line-widths, depending on electronic states and J' quantum numbers, is indicative of number of different predissociation channels. The relationship between line-widths, line-shifts, and signal intensities reveals dissociation mechanisms involving ion-pair to Rydberg state interactions prior to direct or indirect predissociations of Rydberg states. Quantum interference effects are found to be important. Moreover, observed bromine atom (2 + 1) REMPI signals support the importance of Rydberg state predissociation channels. A band system, not previously observed in REMPI, was observed and assigned to the k(3)Π(0)(v' = 0) ←← X transition with band origin 80,038 cm(-1) and rotational parameter B(v('))=7.238 cm(-1).     

Ultrafast decay of superexcited c 4Σu- nlσg v=0,1 states of O2 probed with femtosecond photoelectron spectroscopy

Neutral superexcited states in molecular oxygen converging to the O(2)(+) c (4)Σ(u)(-) ion state are excited and probed with femtosecond time-resolved photoelectron spectroscopy to investigate predissociation and autoionization relaxation channels as the superexcited states decay. The c (4)Σ(u)(-) 4sσ(g) v=0, c (4)Σ(u)(-) 4sσ(g) v=1, and c (4)Σ(u)(-) 3dσ(g) v=1 superexcited states are prepared with pulsed high-harmonic radiation centered at 23.10 eV. A time-delayed 805 nm laser pulse is used to probe the excited molecular states and neutral atomic fragments by ionization; the ejected photoelectrons from these states are spectrally resolved with a velocity map imaging spectrometer. Three excited neutral O* atom products are identified in the photoelectron spectrum as 4d(1)?(3)D(J)°, 4p(1) (5)P(J)° and 3d(1) (3)D(J)° fragments. Additionally, several features in the photoelectron spectrum are assigned to photoionization of the transiently populated superexcited states. Using principles of the ion core dissociation model, the atomic fragments measured are correlated with the molecular superexcited states from which they originate. The 4d(1) (3)D(J)° fragment is observed to be formed on a timescale of 65 ± 5 fs and is likely a photoproduct of the 4sσ(g) v = 1 state. The 4p(1) (5)P(J)° fragment is formed on a timescale of 427 ± 75 fs and correlated with the neutral predissociation of the 4sσ(g) v = 0 state. The timescales represent the sum of predissociation and autoionization decay rates for the respective superexcited state. The production of the 3d(1) (3)D(J)° fragment is not unambiguously resolved in time due to an overlapping decay of a v = 1 superexcited state photoelectron signal. The observed 65 fs timescale is in good agreement with previous experiments and theory on the predissociation lifetimes of the v = 1 ion state, suggesting that predissociation may dominate the decay dynamics from the v = 1 superexcited states. An unidentified molecular state is inferred by the detection of a long-lived depletion signal (reduction in autoionization) associated with the B (2)Σ(g)(-) ion state that persists up to time delays of 105 ps.     

Spectroscopic analysis of the coupled 1(1)Π, 2(3)Σ+ (Ω = 0-, 1), and b(3)Π (Ω = 0±, 1, 2) states of the KRb molecule using both ultracold molecules and molecular beam experiments

We report the spectroscopic characterization of excited electronic states of KRb by combining spectra from molecular beam (MB) experiments with those from ultracold molecules (UM) formed by photoassociation (PA) of ultracold atoms. Spectra involving the 1(1)Π, 2(3)Σ(+), and b(3)Π states in a strongly perturbed region have been identified. This approach provides a powerful method to identify the vibrational levels of the excited electronic states perturbed globally by neighboring electronic states. This is because the two sets of spectra from the UM and the MB experiments probe the same energy region from very different initial electronic states. The UM experiments utilize high v' levels of the a(3)Σ(+) state with large internuclear separations, while the MB experiments utilize low v' levels of the ground X(1)Σ(+) state with near-equilibrium internuclear separations. Only the Ω = 1 levels of the 2(3)Σ(+) and b(3)Π states are observed in the MB spectra, while the Ω = 0(-), 1 levels of the 2(3)Σ(+) state and the Ω = 0(±), 1, 2 levels of the b(3)Π state are observed in the UM spectra.     

C6H and C6D: electronic spectra and Renner-Teller analysis

Rotationally resolved spectra of the B(2)Π - X(2)Π 0(0)(0) electronic origin bands and 11(1)(1) μ(2)Σ-μ(2)Σ vibronic hot band transitions of both C(6)H and C(6)D have been recorded in direct absorption by cavity ring-down spectroscopy through a supersonically expanding planar plasma. For both origin and hot bands accurate spectroscopic parameters are derived from a precise rotational analysis. The origin band measurements extend earlier work and the 11(1)(1) μ(2)Σ-μ(2)Σ vibronic hot bands are discussed here for the first time. The Renner-Teller effect for the lowest bending mode ν(11) is analyzed, yielding the Renner parameters ε(11), vibrational frequencies ω(11), and the true spin-orbit coupling constants A(SO) for both (2)Π electronic states. From the Renner-Teller analysis and spectral intensity measurements as a function of plasma jet temperature, the excitation energy of the lowest-lying 11(1) μ(2)Σ vibronic state of C(6)H is determined to be (11.0 ± 0.8) cm(-1).     

Characterization of C4H in the A2Π and X2Σ+ states by double resonance four-wave mixing

The B(2)Π-X(2)Σ(+) electronic spectrum of C(4)H has been studied by degenerate and double resonance four-wave mixing. The technique identifies vibrational levels in the X(2)Σ(+) ground state. Its sensitivity and unique characteristics permit detection of new levels. The A(2)Π state lying 222 cm(-1) above the X(2)Σ ground state is also observed, confirming the analysis from anion photoelectron spectroscopy but with improved accuracy. Vibrational level determination in the A(2)Π electronic manifold up to 700 cm(-1) above v = 0 is made. A Renner-Teller analysis is carried out for the two lowest bending modes v(6) and v(7) in the A(2)Π state by diagonalization of the effective Hamiltonian matrix. The Renner-Teller parameters ∈(6), ∈(7), and ∈(67), the vibrations ω(6) and ω(7) and the spin-orbit coupling constant A(so) are determined.     

Ab initio configuration interaction study of the B- and C-band photodissociation of methyl iodide

Multireference spin-orbit configuration interaction calculations have been carried out for the valence and low-lying Rydberg states of CH(3)I. Potential energy surfaces along the C-I dissociation coordinate (minimal energy paths with respect to the umbrella angle) have been obtained as well as transition moments for excitation of the Rydberg states. It is shown that the B and C absorption bands of CH(3)I are dominated by the perpendicular (3)R(1),(1)R?(E)←X??A(1) transitions, while the (3)R(2)(E),?(3)R(0(+) )(A(1))←X??A(1) transitions are very weak. It is demonstrated that the bound Rydberg states of the B and C bands are predissociated due to the interaction with the repulsive E and A(2) components of the (3)A(1) state, with the (3)A(1)(E) state being the main decay channel. It is predicted that the only possibility to obtain the I((2)P(3/2)) ground state atoms from the CH(3)I photodissociation in the B band is by interaction of the (3)R(1)(E) state with the repulsive (1)Q(E) valence state at excitation energies above 55,000 cm(-1). The calculated ab initio data are used to analyze the influence of the Rydberg state vibrational excitation on the decay process. It is shown that, in contrast to intuition, excitation of the ν(3) C-I stretching mode supresses the predissociation, whereas the ν(6) rocking vibration enhances the predissociation rate.     

Vibronic coupling in the A2Π and B2Σ+ electronic states of the NCS radical

The spin-rovibronic energy levels of the A(2)Π and B(2)Σ(+) electronic states of thiocyanate radical have been calculated variationally, using high-level ab initio coupled diabatic potential energy surfaces. Computations up to J = 7∕2 have been performed, obtaining all levels with K ≤ 3 (Σ(1/2),Π(1/2,3/2),Δ(3/2,5/2),Φ(5/2,7/2)), for energies up to 2000 cm(-1) above the A(000)(2)Π(3∕2) level. The available experimental data have been critically reviewed in the light of the theoretical findings.     

Dissociation of vibrational state-selected O2(+) Ions in the B(2)Σ(g)¯ state using threshold photoelectron-photoion coincidence velocity imaging

Using the recently developed threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging mass spectrometer (Tang et al. Rev. Sci. Instrum.2009, 80, 113101), dissociation of vibrational state-selected O(2)(+)(B(2)Σ(g)(ˉ), v(+) = 0-6) ions was investigated. Both the speed and angular distributions of the O(+) fragments dissociated from individually vibronic levels of the B(2)Σ(g)(ˉ) state were obtained directly from the three-dimensional time-sliced TPEPICO velocity images. Two dissociation channels, O(+)((4)S) + O((3)P) and O(+)((4)S) + O((1)D), were respectively observed, and their branching ratios were found to be heavily dependent on the vibrational states. A new intersection mechanism was suggested for the predissociation of O(2)(+)(B(2)Σ(g)(ˉ)) ions, especially for dissociation at the energy of the v(+) = 4 level. In addition, the anisotropic parameters for O(+) fragments from different dissociative pathways were determined to be close to zero, indicating that the v(+) = 0-6 levels of B(2)Σ(g)(ˉ) predissociate on a time scale that is much slower than that of molecular rotation.     

Ion-pair dissociation dynamics of H2S in the photon energy range 15.26-15.55 eV

The H(+) velocity map images from the ion-pair dissociation of H(2)S + hν → SH(-)(X(1)Σ(+), υ = 0, 1) + H(+) have been measured at the excitation energies 15.259, 15.395, and 15.547 eV, respectively. The experimental results show that most of the available energies are transformed into the translational energies. The angular distributions of the fragments SH(-)(X(1)Σ(+), υ = 0) indicate that the dissociation occurs via pure parallel transition with limiting anisotropy parameter of +2. Because the ion-pair dissociation usually occurs via the predissociation of Rydberg states, this suggests that the ion cores of the excited Rydberg states have linear geometries. The geometries and electronic structures of the linear H(2)S(+) have been calculated employing the quantum chemistry calculation method at the CASPT2/avqz level. The electronic structures for the ion-pair states have been calculated at the CASSCF/avtz level, which indicates that the equilibrium geometries of the ion-pair states have bent geometries.     

Two-color photodetachment study of the A3Π-X3Σ- origin band of C5H-

The origin band in the electronic transition to the dipole bound excited state of C(5)H(D)(-) anions was measured using two-color photodetachment spectroscopy. The rotational analysis of the partially resolved contour is consistent with a linear structure of the anion in both the ground X(3)Σ(-) and excited A(3)Π electronic states, in contrast with an earlier interpretation. The following spectroscopic constants are inferred for C(5)H(-): T(00) = 19248.0(1), B' = 0.0835(1), B' = 0.0826(2), A'(SO) = -11.96(1), λ(SS)' = 1.97(8) λ(SS)' = 0.24(15). Ab initio calculations at the UHF-MP2 level support the conclusion that C(5)H(-) is linear in the ground state. The experimentally determined ground state rotational constant can be used in the search for the millimeter wave spectrum of C(5)H(-).     

Electronic spectroscopy of the previously unknown arsenic carbide (AsC) free radical

The previously unknown arsenic carbide (AsC) free radical has been identified in the gas phase through a combination of laser-induced fluorescence (LIF), single vibronic level emission, and stimulated emission pumping (SEP) spectroscopy in a supersonic expansion. The As(12)C and As(13)C isotopologues have been detected as products of an electric discharge in mixtures of arsine (AsH(3)) and carbon dioxide ((12)CO(2) or (13)CO(2)) in high pressure argon. The B (2)Σ(+)-X (2)Σ(+) band system was recorded by LIF spectroscopy and emission transitions from the B state down to the ground state and to the low-lying A (2)Π(i) state were observed. High resolution studies of the B-X 0-0 band by LIF and the B-A 0-0 band by SEP spectroscopy enabled a determination of the molecular structures in the three states. Although CN, CP, and AsC have similar X (2)Σ(+) and A (2)Π(i) states, the B (2)Σ(+) state molecular orbital configuration of CP and AsC differs from that of the CN free radical.     

Experimental and theoretical studies of the CN-Ar van der Waals complex

The CN-Ar van der Waals complex has been observed using the B (2)Sigma(+)-X (2)Sigma(+) and A (2)Pi-X (2)Sigma(+) electronic transitions. The spectra yield a dissociation energy of D(0")=102+/-2 cm(-1) and a zero-point rotational constant of B(0")=0.067+/-0.005 cm(-1) for CN(X)-Ar. The dissociation energy for CN(A)-Ar was found to be D(0')=125+/-2 cm(-1). Transitions to vibrationally excited levels of CN(B)-Ar dominated the B-X spectrum, indicative of substantial differences in the intermolecular potential energy surfaces (PESs) for the X and B states. Ab initio PESs were calculated for the X and B states. These were used to predict rovibrational energy levels and van der Waals bond energies (D(0")=115 and D(0')=183 cm(-1)). The results for the X state were in reasonably good agreement with the experimental data. Spectral simulations based on the ab initio potentials yielded qualitative insights concerning the B-X spectrum, but the level of agreement was not sufficient to permit vibronic assignment. Electronic predissociation was observed for both CN(A)-Ar and CN(B)-Ar. The process leading to the production of CN(A,nu=8,9) fragments from the predissociation of CN(B,nu=0)-Ar was characterized using time-resolved fluorescence and optical-optical double resonance measurements.     

Experimental investigation of electronic states of LiCs dissociating to Li(2(2)S) and Cs(5(2)D) atoms

Polarization labeling spectroscopy technique was used to measure excitation spectra of LiCs molecule in the spectral range of 16,000-18,500 cm(-1). Four band systems were observed and assigned to transitions from the ground X(1)Σ(+) state to excited states (4)Ω = 0(+), (5)Ω = 0(+), (5)Ω = 1, and (6)Ω = 1 (in Hund's case (c) notation proper here), the latter three states being fine structure components of the states d(3)Π and e(3)Σ(+), nominally of triplet symmetry. The observed states are characterized spectroscopically and the experimental results are compared with predictions of theoretical calculations, showing accuracy of the theoretical electronic term values better than 100 cm(-1) and of the ω(e) and R(e) constants within 5%.