Dispersed Fluorescence Studies Research Articles

Observation of a new Ω = 1 excited state in LaH

We report the observation of an Ω=1 electronic state with T0=20041.96(1) and T1=21359.1(5)cm−1. This state was identified by analyzing the laser-induced fluorescence spectra of jet-cooled LaH molecule. The spectra showed two new bands having P, R and strong Q branch structures with band origin at 19972.00(2) originated from v=1 and 20041.96(1)cm−1 from v=0 of the X1Σ+ ground state. The molecular constants of the excited state were determined. The dispersed fluorescence study also lead to the observation of Ω=2 spin-orbit component of the low-energy b3Π state, which remain unobserved in earlier studies.

Vibrations of porphycene in the S and S1 electronic states: Single vibronic level dispersed fluorescence study in a supersonic jet

Supersonic jet-isolated porphycene has been studied using the techniques of laser-induced fluorescence excitation, single vibronic level fluorescence, and spectral hole burning, combined with quantum mechanical calculations of geometry and vibrational structure of the ground and lowest electronically excited singlet states. Porphycene is a model for coherent double hydrogen tunneling in a symmetrical double well potential, as evidenced by tunneling splittings observed in electronic absorption and emission. The results led to reliable assignment of low frequency modes in S0 and S1 electronic states. The values of tunneling splitting were determined for ground state vibrational levels. In the case of tautomerization-promoting 2A(g) mode, tunneling splitting values significantly increase with the vibrational quantum number. Mode coupling was demonstrated by different values of tunneling splitting obtained for coexcitation of two or more vibrations. Finally, alternation of relative intensity patterns for the components of 2A(g) tunneling doublet observed for excitation and emission into different vibrational levels suggests that the energy order of levels corresponding to (+) and (-) combinations of nuclear wave functions is different for even and odd vibrational quantum numbers.

Dark state vibronic coupling in the Ã(2Π) ←X̃(2Σ+) band of ethynyl radical via high resolution infrared absorption spectroscopy

The high resolution infrared spectrum for the à ((2)Π) ← ͠X ((2)Σ(+)) origin band of jet-cooled ethynyl radical (C(2)H) in the gas phase is reported, which exhibits a strong, parity-specific local perturbation in the upper (2)Π(1/2) state. Based on revised parity assignments of the levels, the perturbing state is unambiguously determined to be (2)Σ(+) symmetry, and thus coupled to the à ((2)Π) state by ΔK = ±1 Coriolis interactions. By incorporating Σ-Π Coriolis coupling into the unperturbed Hamiltonian (containing only rotational, spin-rotational, spin-orbit, and lambda-doubling contributions), we are now able to fit the observed (2)Π-(2)Σ(+) origin band to a sub Doppler experimental uncertainty of 15 MHz (0.0005 cm(-1)). In addition, the observation of pairs of transitions to mixed states permits determination of the band origin (ν(pert)) and rotational constant (B(pert)) for the "dark"(2)Σ(+) state, which prove to be in remarkably quantitative agreement with full vibronic predictions of Tarroni and Carter as well as UV dispersed fluorescence studies of Hsu et al. This represents an important benchmark in mapping out non-Born-Oppenheimer vibronic interactions and energy level structure in a polyatomic combustion radical system, an understanding of which will be key to modeling chemical reactions in both terrestrial and astronomical environments.

Laser-induced fluorescence and dispersed fluorescence studies of the donor–acceptor system 4-amino 3-methyl benzoic acid methyl ester and its solvated clusters: Evidence of excited-state charge-transfer reaction

Laser-induced fluorescence (LIF) excitation and dispersed fluorescence (DF) spectra of 4-amino 3-methyl benzoic acid methyl ester (AMBME) and its solvated clusters with solvents such as methanol, water and acetonitrile have been investigated in a supersonic expansion. Spectral signature supports the presence of two conformers in the cooled jet which is in line with theoretical calculations. In addition to structured local emission from the Franck Condon excited state, the molecule AMBME shows red-shifted broad emission from the state attributed to the close-lying charge transfer (CT) state, which is facilitated by exciting low-frequency modes. The molecule readily forms clusters with different solvents and the clusters’ electronic excitation bands appear in the low-energy side of the transition origin of the bare molecule. Excitation of the clusters leads to the appearance of red-shifted solvent-polarity dependent CT emission.

Vibrational predissociation in weakly-bound clusters: A dispersed fluorescence study of toluene rare-gas complexes

The vibrational predissociation (VP) dynamics of toluene–Ar and toluene–Ne complexes have been investigated using single vibronic level fluorescence spectroscopy. For several levels, comparison of the dispersed fluorescence spectra with those from monomer levels permits the identification of the final monomer vibrational level. Pumping of the low-lying S 1 13 0 1 / 24 0 1 25 0 1 ¯ or 37 0 1 ¯ transitions in toluene–Ar, for example, gives rise to efficient VP leaving the resulting toluene monomer in its vibrationless S 1 state. By contrast, in the case of toluene–Ne, VP following 37 1 ¯ level pumping leaves the toluene molecule in an excited 16 1 level which is optically inaccessible from the ground state.

A dispersed fluorescence study of the [formula omitted] transition

A dispersed fluorescence study of the C O 2 + B ˜ ( 0 0 1 ) Σ g + 2 → X ˜ ( 0 0 1 ) Π u 2 transition has been performed in the photon energy range 18.2–19.9 eV. The fluorescence excitation spectrum exhibits prominent structure ascribed to two electric dipole forbidden Rydberg series, both of which involve a single excitation of the asymmetric stretching mode, converging onto the C ˜ ( 0 0 1 ) Σ u + 2 ionisation limit. The series, whose presence can be attributed to vibronic coupling, may be due to 4σ g → nsσ g ( δ ∼ 0.93) and 4σ g → nd ( δ ∼ 0.38) transitions.

Vibronic spectroscopy of unsaturated transition metal complexes: CrC2H, CrCH3, and NiCH3

Vibronically resolved resonant two-photon ionization and dispersed fluorescence spectra of the organometallic radicals CrC(2)H, CrCH(3), and NiCH(3) are reported in the visible and near-infrared wavelength regions. For CrC(2)H, a complicated vibronic spectrum is found in the 11 100-13 300 cm(-1) region, with a prominent vibrational progression having omega(e) (')=426.52+/-0.84 cm(-1), omega(e) (')x(e) (')=0.74+/-0.13 cm(-1). Dispersed fluorescence reveals a v(")=1 level of the ground state with DeltaG(1/2) (")=470+/-20 cm(-1). These vibrational frequencies undoubtedly pertain to the Cr-C(2)H stretching mode. It is suggested that the spectrum corresponds to the A (6)Sigma(+)<--X (6)Sigma(+) band system, with the CrC(2)H molecule being linear in both the ground and the excited state. The related CrCH(3) molecule displays a vibronic spectrum in the 11 500-14 000 cm(-1) region. The upper state of this system displays six sub-bands that are too closely spaced to be vibrational structure, but too widely separated to be K structure. It is suggested that the observed spectrum is a (6)E<--X (6)A(1) band system, analogous to the well-known B (6)Pi<--X (6)Sigma(+) band systems of CrF and CrCl. The ground state Cr-CH(3) vibration is characterized by omega(e) (")=525+/-17 cm(-1) and omega(e) (")x(e) (")=7.9+/-6 cm(-1). The spectrum of NiCH(3) lies in the 16 100-17 400 cm(-1) range and has omega(e) (')=455.3+/-0.1 cm(-1) and omega(e) (')x(e) (')=6.60+/-0.03 cm(-1). Dispersed fluorescence studies provide ground state vibrational constants of omega(e) (")=565.8+/-1.6 cm(-1) and omega(e) (")x(e) (")=1.7+/-3.0 cm(-1). Again, these values correspond to the Ni-CH(3) stretching motion. (c) 2004 American Institute of Physics.

The spectrum of singlet SiH2

We report a theoretical study of the two lowest singlet electronic states ([Formula: see text]1A1 and Ã1B1) of silylene SiH2. These states become degenerate as a 1Δg state at linear configurations and are subject to the Renner effect. In ab initio calculations we have determined the potential energy and dipole moment surfaces for each state, and the transition moment surface between the states. Parameterized analytical functions have been fitted through the various sets of ab initio points, and the parameter values obtained for the potential energy surfaces have been further refined in fittings to experimental spectroscopic data. In these latter fittings, we use as input data experimentally derived energy differences together with ab initio points. In this manner, we achieve refined potential energy surfaces that behave reasonably also in regions of configuration space that are not sampled by the wavefunctions of the states for which experimentally derived energies are available. The calculation of rovibronic energies, the fittings to experimentally derived energies, and simulations of Ã1B1 → [Formula: see text]1A1 emission spectra of SiH2 have been carried out with the RENNER program system. The higher excited vibrational states of [Formula: see text]1A1 SiH2 form polyads of heavily interacting states and many polyad states have been observed in dispersed fluorescence studies. The present theoretical work shows that owing to the heavy interaction between the states in the polyads, it is difficult to obtain unambiguous assignments for them.Key words: silylene, RENNER, ab initio.PACS Nos.: 31.15.Ar, 33.20.Wr, 33.20.Ea

Dispersed fluorescence spectroscopy of AlNi, NiAu, and PtCu

Dispersed fluorescence studies of AlNi, NiAu, and PtCu have been performed, providing spectroscopic information about the ground and low-lying excited electronic states. Vibrational frequencies are reported for the ground X 2Δ5/2 state of all three molecules. In the case of AlNi, fluorescence to all five of the states originating from the 3dNi93sAl2σ2 manifold has been observed. For both NiAu and PtCu, fluorescence to two low-lying excited states in addition to the ground state was observed. Relative energies, vibrational constants, and, when possible, Ω values of these states are reported. Comparisons of the measured electronic states to the predictions of a ligand-field plus spin–orbit model are also provided, along with a comparison of the electronic structure of PtCu to that of PtH.

Vibrational predissociation in aminophenol–argon 1 complex

Resonantly enhanced multiphoton ionization and dispersed fluorescence studies, as well as ab initio calculations on the van der Waals complex of p-aminophenol with argon are reported. From the absence of features in the REMPI spectrum beyond 6a, the binding energy of the complex was estimated. The dispersed fluorescence spectra of the complex at various excitations were used to obtain information about the dynamics of vibrational predissociation. The character of the dispersed fluorescence spectra and the VP rate constants obtained using the serial model were intermediate between the aniline–argon and aniline–CH 4 cases.

Identification of New Low-Lying Electronic States of the FeF Radical

Dispersed fluorescence studies on the 6Π–X6Δ and 6Φ−X6Δ systems of the FeF radical have resulted in the observation of vibrational progressions for transitions to the X6Δ state as well as at least two previously unobserved electronic states about 5000 cm−1 above the ground state. The states are assigned as the A6Π and B6Σ+ electronic states. The spin components of both electronic states were found to be heavily perturbed resulting in uneven splittings between them. A third, weak series was also observed but could not be assigned. The (0,0) band of the 6Π7/2−B6Σ+5/2 transition at 398 nm was observed in absorption by laser induced fluorescence and its rotational structure was assigned. The spectra obtained were weak because of a poor population of the B6Σ+ state by the reaction used to form FeF. The levels were found to be markedly perturbed at high J values. Attempts were made to fit the data on the 6Π7/2–B6Σ5/2+ system to an effective Hamiltonian, but the presence of perturbations meant that the system is not well described by such a model.

Optical spectroscopy of tungsten carbide (WC)

Resonant two-photon ionization spectroscopy has been used to study the diatomic transition-metal carbide, WC. A low-resolution scan revealed a five-member vibrational progression beginning with the 0-0 band at 17 585 cm−1. Analysis of this progression yielded a vibrational frequency of ωe′(184W12C)=752.6(4.9) cm−1 and a bond length of re′(184W12C)=1.747(4) Å. Several unassigned bands were also rotationally resolved and analyzed. All of the observed bands are Ω′=2←Ω″=1 transitions, confirming the predicted ground state of Δ13 arising from a 14σ28π415σ24δ116σ1 configuration. The measured line positions in these bands were simultaneously fitted to provide B0″=0.509 66(10) cm−1 for W12184C, corresponding to r0″(184W12C)=1.713 5(2) Å. These values are corrected for spin-uncoupling effects in the ground state and represent our best estimate of the true bond length of WC. Dispersed fluorescence studies provide the ground-state vibrational constants of ωe=983(4) cm−1 and ωexe=11(1) cm−1, and have also permitted the low-lying [1.2] 3Δ2 and [4.75] states to be located and characterized. These results on WC are discussed in relation to the isovalent molecule MoC and other transition-metal carbides.

Laser-induced dispersed vibration–rotation fluorescence: a new technique to study molecular states

Two sensitive high-resolution laser absorption techniques, intra-cavity photo-acoustic and cavity ring-down spectroscopy (CRDS), are discussed in the context of laser-induced dispersed fluorescence studies of molecular overtone states. These absorption methods provide spectral data of comparable quality. The laser-induced dispersed fluorescence experiment offers new possibilities when applied to vibration–rotation states within the ground electronic states. Particularly, some new states in symmetrical molecules not accessible by one-photon absorption spectroscopy can be reached. Interesting collisional effects including nuclear spin conversion can also be investigated.

Dispersed fluorescence studies of linear LiOLi: a strongly bound, but very `floppy' ionic molecule

By dispersing laser-induced fluorescence (LIF) due to transitions from bent, excited LiOLi( 1 B 1, K ′=1) rovibrational states to linear ground-state LiOLi[ X ̃ 1Σ g +(ν 1″,ν 2″,ν 3″)] vibrational states, we have determined 37 ( ν 1″, ν 2″, ν 3″) vibrational eigenstate energies, each to an accuracy of about ±10 cm −1 . An eight-term power series fit of the energies results in estimates of the ω 1″, ω 2″, ω 3″ fundamental frequencies, the χ 11″, χ 22″ anharmonicity constants, and the χ 12″, χ 23″, χ 13″ coupling constants. For such a strongly bound ionic molecule (Li 1+O 2−Li 1+), LiOLi is remarkably floppy, with a `light-atom' ω 2″ bending frequency of only 122±4 cm −1 .

The FeH radical: rotational assignment of the a6A Δ(ν = 2) and b6II (ν = 1) levels

A dispersed fluorescence study of the FeH radical at near ambient (∼450 K) temperature has identified the g6Φ-a 6δ (0, 2) and g6Φ-b6II (0, 1) bands in the orange region of the spectrum. The corresponding bands with upper state e6II are located at about 640nm in the red. High resolution measurements in LIF have enabled the assignment of around 275 lines to these four transitions. Analysis of these bands has provided a second vibrational interval for the a6δ state and a first vibrational interval for the b6II state as 1536cm−1 and 1542cm−1, respectively. Term values have also been measured for a set of levels with a tentative assignment to c4Φ9/2 (ν = 1).

Electronic structure of the 4d transition metal carbides: Dispersed fluorescence spectroscopy of MoC, RuC, and PdC

Dispersed fluorescence studies of the diatomic molecules MoC, RuC, and PdC are reported. New states identified in MoC and RuC are the […]2δ112σ1, 3,1Δ2 states and the […]2δ312σ1, 1Δ2 state, respectively. Five states are observed by dispersed fluorescence in PdC. The ground state is found to be […]2δ412σ2, 1Σ+, with the […]2δ412σ16π1, 3ΠΩ manifold of states lying about 2500 cm−1 above the ground state. The [17.9]Ω=1 state of PdC is also identified as […]2δ412σ113σ1, 3Σ+(Ω=1), corroborating recent results of resonant two photon ionization spectroscopy studies. The spin-orbit interactions of these molecules are analyzed to deduce the composition of the molecular orbitals, and comparisons are made to ab initio theory when possible. An examination of the trends in bond energy, bond length, and vibrational frequency among the 4d transition metal carbides is also provided.

Spectroscopy of van der Waals molecules: Isomers and vibrational predissociation

The inert-gas-halogen complexes have been studied for several decades by jet spectroscopy. Much of the seemingly bizarre behavior has become understandable in terms of two virtually isoenergetic isomer forms. The recently recognized linear isomer of Ar–I2 has a virtually continuous B ¬ X excitation spectrum. It also undergoes a very rapid vibrational predissociation, and suffers no electronic quenching from the B state. The well-known T-shaped isomer shows slow vibrational predissociation, which is competitive with electronic quenching. The quenching distorts the vibrational distribution of the I2 B state photofragments, consequently leading to a false estimation of the T-shaped Ar–I2 (B) state dissociation energy. The binding energies for the T-shaped Ar–I2 (X) and Ar–I2 (B) are unambiguously determined from the recent dispersed fluorescence study, which are also in good accord with the ab initio calculation. We discuss aspects of pure vibrational laser-induced fluorescence of hydrogen fluoride complexes. We contrast the behavior of Ar–HF with Ne–HF and present new results for the vHF = 3 level of Ne–HF. PACS Nos.: 33.80Gj, 34.30th

Observation of Coriolis Coupling between ν2 + 4ν4 and 7ν4 in Acetylene X̃1Σ+g by Stimulated Emission Pumping Spectroscopy

Stimulated emission pumping (SEP) spectroscopy has been used to examine a low energy region (Evib ≈ 4400 cm−1) of X̃1Σ+g acetylene at higher resolution than was possible in previous dispersed fluorescence studies. The expected bright state, ν2 + 4ν4, is observed to be coupled to the nearly degenerate 7ν4 state by a Coriolis mechanism. A least-squares analysis yields values for zero-order vibrational energies, rotational constants, and a Coriolis-coupling coefficient that are all consistent with expectations. Calculated relative intensities of SEP transitions, accounting for interference due to axis-switching effects, are also consistent with observations. Implications of the observed Coriolis resonance with regard to global acetylene vibrational dynamics are also discussed.

Dynamics of linear and T-shaped Ar–I2 dissociation upon B←X optical excitation: A dispersed fluorescence study of the linear isomer

We report the dispersed fluorescence spectra of the linear and the previously well-studied T-shaped isomers of Ar–I2 following B←X optical excitation for vpump=16–26, below the I2 dissociation limit. The linear isomer has a continuum excitation spectrum. For excitation at the highest pumping energy (vpump=26), the product vibrational state distribution is nearly identical to that observed for excitation above the I2(B) dissociation limit; it shows a broad, nearly Gaussian distribution of I2(B) vibrational states, with about 22% of the available excess energy deposited in translation of the Ar+I2. This gives direct evidence that the “one-atom cage” effect seen above the I2(B) dissociation limit is attributable to the linear Ar–I2 isomer. The product vibrational state distribution becomes increasingly Poisson for decreasing excitation energies, and only about 7% of the excess energy is deposited in translation for vpump=16. The bond energy in the linear isomer is determined from the spectra, 170(±1.5)⩽D0″(linear Ar–I2(X))⩽174(±1.5) cm−1. A bond energy of D0″(T-shaped Ar–I2(X))=142±15 cm−1 is estimated based on the linear to T-shaped population ratio observed in the beam, which is about 90 cm−1 smaller than that determined from fluorescence spectra. We suggest that electronic quenching in the T-shaped isomer is nearly 100% for the highest vibrational level produced by vibrational predissociation.

Resonant two-photon ionization spectroscopy of jet-cooled PdC

The first optical investigation of the spectra of diatomic PdC has revealed that the ground state has Ω=0+, with a bond length of r0=1.712 Å. The Hund’s case (a) nature of this state could not be unambiguously determined from the experimental data, but dispersed fluorescence studies to be reported in a separate publication, in combination with a comparison to theoretical calculations, demonstrate that it is the 2δ4 12σ2, 1Σ0++ state, which undergoes spin–orbit mixing with a low-lying 2δ4 12σ1 6π1, 3Π0+ state. An excited 3Σ+ state with re=1.754±0.003 Å (r0=1.758±0.002 Å) and ΔG1/2=794 cm−1 is found at T0=17 867 cm−1. Although only the Ω=1 component of this state is directly observed, the large hyperfine splitting of this state for the 105Pd 12C isotopomer implies that an unpaired electron occupies an orbital that is primarily of 5s character on Pd. Comparison to ab initio calculations identifies this state as 2δ4 12σ1 13σ1, 3Σ1+. To higher wavenumbers a number of transitions to states with Ω=0+ have been observed and rotationally analyzed. Two groups of these have been organized into band systems, despite the clear presence of homogeneous perturbations between states with Ω=0+ in the region between 22 000 and 26 000 cm−1.