Stimulated Emission Pumping Spectroscopy Research Articles

Communication: Observation of local-bender eigenstates in acetylene.

We report the observation of eigenstates that embody large-amplitude, local-bending vibrational motion in acetylene by stimulated emission pumping spectroscopy via vibrational levels of the S1 state involving excitation in the non-totally symmetric bending modes. The N(b) = 14 level, lying at 8971.69 cm(-1) (J = 0), is assigned on the basis of degeneracy due to dynamical symmetry breaking in the local-mode limit. The level pattern for the N(b) = 16 level, lying at 10 218.9 cm(-1), is consistent with expectations for increased separation of ℓ = 0 and 2 vibrational angular momentum components. Increasingly poor agreement between our observations and the predicted positions of these levels highlights the failure of currently available normal mode effective Hamiltonian models to extrapolate to regions of the potential energy surface involving large-amplitude displacement along the acetylene ⇌ vinylidene isomerization coordinate.

Towards a global model of spin-orbit coupling in the halocarbenes.

We report a global analysis of spin-orbit coupling in the mono-halocarbenes, CH(D)X, where X = Cl, Br, and I. These are model systems for examining carbene singlet-triplet energy gaps and spin-orbit coupling. Over the past decade, rich data sets collected using single vibronic level emission spectroscopy and stimulated emission pumping spectroscopy have yielded much information on the ground vibrational level structure and clearly demonstrated the presence of perturbations involving the low-lying triplet state. To model these interactions globally, we compare two approaches. First, we employ a diabatic treatment of the spin-orbit coupling, where the coupling matrix elements are written in terms of a purely electronic spin-orbit matrix element which is independent of nuclear coordinates, and an integral representing the overlap of the singlet and triplet vibrational wavefunctions. In this way, the structures, harmonic frequencies, and normal mode displacements from ab initio calculations were used to calculate the vibrational overlaps of the singlet and triplet state levels, including the full effects of Duschinsky mixing. These calculations have allowed many new assignments to be made, particularly for CHI, and provided spin-orbit coupling parameters and values for the singlet-triplet gaps. In a second approach, we have computed and fit full geometry dependent spin-orbit coupling surfaces and used them to compute matrix elements without the product form approximation. Those matrix elements were used in similar fits varying the anharmonic constants and singlet-triplet gap to reproduce the experimental levels. The derived spin-orbit parameters for carbenes CHX (X = Cl, Br, and I) show an excellent linear correlation with the atomic spin-orbit constant of the corresponding halogen, indicating that the spin-orbit coupling in the carbenes is consistently around 14% of the atomic value.

Picosecond rotationally resolved stimulated emission pumping spectroscopy of nitric oxide

Stimulated emission pumping (SEP) experiments were performed on the nitric oxide molecule in a flow cell environment using lasers with pulse widths of 17–25 ps. A lambda excitation scheme, or ‘‘pump–dump” arrangement, was employed with the pump laser tuned to the T 00 vibronic band origin ( λ pump = 226.35 ( 1 ) nm ) of the A 2 Σ +( v′ = 0, J′) ← X 2 Π 1/2( v″ = 0, J″) and the dump laser scanned from 246–248 nm within the A 2 Σ +( v′ = 0, J′) → X 2 Π 1/2( v″ = 2, J″) transition. The rotationally resolved SEP spectra were measured by observing the total fluorescence within the A 2 Σ +( v′ = 0, J′) → X 2 Π 1/2( v″ = 1, J″) transition between 235 nm and 237.2 nm while scanning the dump laser wavelengths. Multiple rotational states were excited due to the broad laser bandwidth. Measurements showed that the resolved rotational structure depended on the energy and bandwidth of the applied pump and dump laser pulses. Analysis of the observed fluorescence depletion signals yielded an average percent fluorescence depletion of about 19% when λ pump = 226.35 ( 1 ) nm and λ dump = 247.91 ( 1 ) nm . This value reflects the percent transfer of the NO population from the A 2 Σ +( V′ = 0, J′) excited electronic state to the X 2 Π 1/2( v″ = 2, J″) ground electronic state. The maximum expected depletion is 50% in the limit of dump saturation. Selective excitation of NO at the bandhead provides good spectral discrimination from the background emission and noise and unambiguously confirms the identity of the emitter.

High resolution study of spin-orbit mixing and the singlet-triplet gap in chlorocarbene: Stimulated emission pumping spectroscopy of CH35Cl and CD35Cl

We report on high resolution studies of spin-orbit mixing and the singlet-triplet gap in a prototypical halocarbene, CHCl, using stimulated emission pumping (SEP) spectroscopy from the A (1)A(") state. Results are reported for two isotopomers, CH(35)Cl and CD(35)Cl. We have obtained rotationally resolved spectra for the majority of X (1)A(') levels lying between 0 and 6000 cm(-1) above the zero-point level that were previously observed under low resolution in single vibronic level emission studies and several new levels that were previously unobserved or unresolved. In addition, SEP spectra were obtained for six a (3)A(") levels in CH(35)Cl and three levels in CD(35)Cl. The derived term energies and rovibrational parameters of the X (1)A(') and a (3)A(") states are in good agreement with theory. The a (3)A(") triplet spin-spin parameter is vibrational state dependent, and dominated by a second-order contribution from spin-orbit coupling with nearby X (1)A(') levels; it therefore provides a sensitive probe of spin-orbit mixing in this system. An analysis of three pairs of interactions between specific a (3)A(") and X (1)A(') levels in CH(35)Cl affords a pure electronic spin-orbit coupling element of 150 cm(-1), in good agreement with theoretical expectations. The derived singlet-triplet gaps, which are the most precise determined to date for any carbene, are compared with the predictions of high level ab initio theory.

Three-dimensional potential energy surface of the Ar–OH(Πi2) complex

Pure rotational transitions in the ground state for Ar-OH and Ar-OD [Y. Ohshima et al., J. Chem. Phys. 95, 7001 (1991) and Y. Endo et al., Faraday Discuss. 97, 341 (1994)], those in the excited states of the OH vibration, nu(s)=1 and 2, observed by Fourier-transform microwave spectroscopy in the present study, rotation-vibration transitions observed by infrared-ultraviolet double-resonance spectroscopy [K. M. Beck et al., Chem. Phys. Lett. 162, 203 (1989) and R. T. Bonn et al., J. Chem. Phys. 112, 4942 (2000)], and the P-level structure observed by stimulated emission pumping spectroscopy [M. T. Berry et al., Chem. Phys. Lett. 178, 301 (1991)] have been simultaneously analyzed to determine the potential energy surface of Ar-OH in the ground state. A Schrodinger equation, considering all the freedom of motions for an atom-diatom system in the Jacobi coordinate, R, theta, and r, was numerically solved to obtain energies of the rovibrational energy levels using the discrete variable representation method. A three-dimensional potential energy surface is determined by a least-squares fitting. In the analysis the potential parameters, obtained by ab initio calculations at the RCCSD(T) level of theory with a set of basis functions of aug-cc-pVTZ and midbond functions, are used as initial values. The determined intermolecular potential energy surface and its dependence on the OH monomer bond length are compared with those of an isovalent radical complex, Ar-SH.

Rotational state-dependent mixings between resonance states of vibrationally highly excited DCO (X2A').

Rotational state-dependent mixings between highly excited resonance states of DCO (X (2)A(')) were investigated by stimulated emission pumping spectroscopy via a series of intermediate rotational levels in the B (2)A(') electronic state of the radical. Two examples for such interactions, between pairs of accidentally nearly degenerate vibrational states at energies of E(v) approximately 6450 and E(v) approximately 10 060 cm(-1), respectively, were analyzed in detail. Deperturbations of the measured spectra provided the zeroth-order vibration-rotation term energies, widths, and rotational constants of the states and the absolute values of the vibrational coupling matrix elements. The coupled states turned out to have very different A rotational constants so that their mixings switch on or off as they are tuned relative to each other as function of the K(a) rotational quantum number. The respective zeroth-order states could be assigned to different interlaced vibrational polyads. Thus, when two states belonging to different polyads are accidentally nearly isoenergetic, even very weak interpolyad interactions may start to play important roles. The derived interpolyad coupling elements are small compared to the typical intrapolyad coupling terms so that their influences on the vibrational term energies are small. However, large effects on the widths (i.e., decay rates) of the states can be observed even from weak coupling terms when a narrow, long-lived state is perturbed by a broad, highly dissociative state. This influence contributes to the previously observed strong state-to-state fluctuations of the unimolecular decay rates of the DCO radical as function of vibrational excitation. Similar mechanisms are likely to promote the transition to "statistical" rates in many larger molecules.

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.

Unimolecular dissociation dynamics of highly vibrationally excited DCO(X̃ 2A). I. Investigation of dissociative resonance states by stimulated emission pumping spectroscopy

The vibrational level structure and unimolecular dissociation dynamics of highly vibrationally excited X̃ 2A DCO were investigated using the method of stimulated emission pumping spectroscopy (SEP). Single vibration-rotation states were probed with excitation energies up to E(X̃)=18 200 cm−1, ≈12 700 cm−1 above the asymptotic D-CO dissociation limit. The vibrational level structure of the molecule was found to be determined by distinctive polyads arising from a 1:1:2 resonance between the CD stretching, CO stretching, and DCO bending vibrations. Anharmonic coupling mechanisms give rise to considerable level mixings, especially regarding the CD and CO stretching motion. Thus, only a minority of vibrational states can be unambiguously assigned. The spectral line shape profiles of ≈100 highly excited “resonance states” in the continuum above the D-CO dissociation limit were measured at high resolution. The profiles are homogeneously broadened. The unimolecular decay rates, obtained from the observed line widths, were observed to fluctuate by more than two orders of magnitude in a strikingly state specific manner. The decay rates on average increase with increasing vibrational excitation energy. The state resolved experimental data are compared to predictions of the microcanonical specific unimolecular rate coefficients calculated from different statistical models. Serious problems were encountered considering the calculation of the density of states of the molecules in the continuum region of the potential energy surface regarding the contribution of the disappearing oscillator. Despite tentative corrections, the calculated rate coefficients were to too high by one to two orders of magnitude. Overall, the unimolecular dynamics of DCO appears to conform to an intermediate case between the strictly vibrationally “mode specific” and the “statistical” limits.

Effect of polarization on stimulated emission pumping spectroscopy of the B 3Π0+u−X 1Σg + system of jet-cooled Br 2 via two-colour resonant four-wave mixing

Spectra of the B 3 Π 0 + u −X 1 Σ g + system of gaseous Br 2 with stimulated-emission pumping in the static gas cell and in a jet were measured via two-colour resonant four-wave mixing technique. The combination of jet-cooling and double-resonance selectivity resulted in only two rotational resonances for each wavelength of pump (grating) beams employed. The intensity, linewidth and the S/N ratio of the rovibronic transitions were strongly influenced by the polarizations of the beams. Intensities of RR lines relative to those of RP lines in various polarization schemes agree with theory.

Observation of the “isomerization states’’ of HCP by stimulated emission pumping spectroscopy: Comparison between theory and experiment

Highly excited vibrational states of HCP X̃ 1∑+ were investigated by stimulated emission pumping spectroscopy. Two distinct families of vibrational states were observed in the 13400–17500 cm−1 energy region. One of them is “normal-mode-type” states, well characterized by polyads involving the bending (ω2) and CP stretching (ω3) vibrations. The other is delocalized “isomerization states” whose existence has been predicted by a recent theoretical study [J. Chem. Phys. 104, 10055 (1996)]. The present spectroscopic observations agree well with the theoretical predictions.

Stimulated emission pumping spectroscopy via two-color resonant four-wave mixing

We present a combined theoretical and experimental study of the application of two-color resonant four-wave mixing (RFWM) to stimulated emission pumping (SEP) spectroscopy. The theoretical approach employs time-independent, diagrammatic perturbation theory and a spherical tensor analysis in an extension of a recent treatment of degenerate four-wave mixing [Williams, Zare, and Rahn, J. Chem. Phys. 101, 1072 (1994)]. The resulting signal expression for two-color RFWM separates the molecular properties from purely laboratory-frame factors determined by the polarizations of the input beams and the rotational branch types of the SEP PUMP and DUMP transitions. This expression is valid in the limit of weak fields and for molecules in which the total angular momentum (omitting nuclear spin) is a good quantum number. In addition, we demonstrate that the spectral response for tuning the DUMP laser is a simple Lorentzian in free-jet experiments. We test our theoretical results and demonstrate the applicability of RFWM-SEP to jet-cooled, transient species in experiments on C3 and HCO. Using the well-studied à 1Πu–X̃ 1Σ+g system of C3, we illustrate and compare the two possible schemes for RFWM-SEP. These are defined as ω1=ω2 (PUMP) and ω3=ω4 (DUMP) or ω1=ω4 (PUMP) and ω2=ω3 (DUMP), where ω1, ω2, and ω3 are the input frequencies and ω4 is the signal frequency. Using the B̃ 2A′–X̃ 2A′ system of HCO, we obtain RFWM-SEP spectra that probe ground-state vibrational resonances lying above the low threshold for dissociation to H+CO. Varying the polarization of the input beams or PUMP rotational branch produce dramatic effects in the relative intensities of rotational lines in the RFWM-SEP spectra of HCO; these effects are well-described by our theoretical analysis. Finally, RFWM-SEP spectra of HCO resonances that are homogeneously broadened by dissociation are consistent with the theoretically predicted Lorentzian line shape; the full widths for these levels are in good agreement with those determined via unsaturated fluorescence depletion SEP.

Stimulated emission pumping spectroscopy of2Σ and2π vibronic levels in [Xtilde](v1v20)2π NCO

We report measurements of 2Σ symmetry rovibronic levels in the (υ 1, υ 2, υ 3)= (110)/(030) Fermi-resonant polyad and the 040 μ 2π states of NCO. The levels were detected by the technique of stimulated emission pumping spectroscopy in a supersonic free jet expansion following excitation of rotational lines in the A(100) 2Σ+−[Xtilde](000) 2π transition of the radical. Combining the new measurements and previously published data we have a complete set of experimental data for 2Σ and 2π vibronic levels of [Xtilde](v1v2 0) 2π NCO for v 1⪯2, v 2⪯4 and for the (010) 2Δ vibronic states. A fairly complete effective Hamiltonian is described and the parameters in it are fitted to a sample representative of all these data. We successfully fit the data to close to the experimental measurement accuracy and derive a unique set of harmonic and anharmonic vibrational terms. The results are interpreted in terms of the electronic structure and vibrational potential energy surface of the radical.

Molecular beam stimulated emission pumping spectroscopy of propynal

Stimulated emission pumping (SEP) experiments were performed on propynal (H–C≡C–CHO). The SEP spectra probed the overtones of the pure C=O stretch (ν4) and a combination of the C=O stretch with the C≡C–C bend (ν9). Term energies for J≤8, K≤2 of the vibrations 42, 43, 44, 45, 4391 as well as two unidentified perturbing vibrational states were obtained. The SEP spectra show simple rotational structure that is assignable by means of a near prolate asymmetric top model. Rotational constants were obtained for all the vibrational states measured. Spectroscopic perturbations are observed for ν4=2,4,5 as either anomalous rotational constants, anomalous energy level shifts, or as extra spectral transitions. Complex state mixing is not observed at the current spectroscopic resolution (0.04 cm−1) even though vibrational state densities as high as 400 states per cm−1 were probed. The propynal SEP results are compared to other SEP experiments on aldehyde containing molecules and inferences are made.

Study of Renner–Teller, spin–orbit, and Fermi-resonance interactions in X̃ 2Π (v1v20) levels of NCO by stimulated emission pumping spectroscopy

We report extensive measurements of rovibronic levels in the 020/100 and 040/120/200 Fermi-resonant polyads of NCO. The levels were accessed by the technique of stimulated emission pumping spectroscopy in a supersonic free jet expansion following excitation of bands in the à 2Σ+−X̃ 2Π spectrum of the radical. The data were analyzed in terms of an effective Hamiltonian which explicitly included reference to all the possible vibronic levels of 2Π symmetry in the polyad under consideration. The effects of levels outside these were treated by perturbation theory. The model was successful in fitting the data close to the experimental measurement precision and the resulting parameters were interpreted in terms of the harmonic and anharmonic vibrational terms. The effective Renner–Teller coupling parameter in NCO varied with vibrational level; however, parameters describing the Fermi-resonance interaction were found to be constant for all the levels investigated.

Efficient state-specific preparation of highly vibrationally excited 15N 18O

Stimulated emission pumping spectroscopy of highly vibrationally excited states of 15N 18O was performed using a tunable argon fluoride laser to access the B state. Stimulated emission resonances from the B state back to v″ = 7, 8, 10, 11, 14, 16, 17, 19, 20, 22, 23, and 24 of the ground electronic state were observed and accurate vibrational state specific molecular constants were derived. The vibrational quantum number dependence of each molecular constant was fit to a polynomial expansion. Rydberg-Klein-Rees analysis was performed in order to derive Franck-Condon factors and R-centroids for the B- X system of 15N 18O.

Vibrationally highly excited acetylene as studied by dispersed fluorescence and stimulated emission pumping spectroscopy: Vibrational assignment of the feature states

The dispersed fluorescence (DF) and stimulated emission pumping (SEP) spectra of acetylene originating from single rovibronic levels of the à 1Au state were measured with resolutions of 30 and 0.5 cm−1, respectively, in order to examine the vibrational level structure of the electronic ground X̃ 1Σ+g state. The SEP spectra revealed that the number of vibrational levels under each peak in the DF spectra increases with vibrational energy from a single vibrational level below 8000 cm−1 to as many as ten vibrational levels above 16 500 cm−1. Taking account of the fact that a peak in the DF spectrum in the high energy region is composed of more than one level, a DF peak is called a feature state (or a feature). In the DF spectra from two trans-bending levels (v3=2 and 3) of the à state a total of 140 DF features between 5 700 and 21 200 cm−1 were detected and long progressions in the trans bend (v″4=6 –18) and CC stretch (v■2=0 –6) were identified. Below 14 000 cm−1, 26 out of the 50 observed features were unambiguously assigned to these two modes and represented by a second order anharmonic expansion within the ∼20 cm−1 experimental error. At least three additional trans-bend progressions built on excitation in third vibrational mode were identified. Possible assignments of the third mode to the CH stretch (ν″1) and the cis bend (ν■5) are compared. The Darling–Dennison (DD) resonance between the two degenerate bending modes (trans and cis) was proposed as a mechanism to lend Franck–Condon (FC) intensity to the ν″5 mode. The vibrational analysis of the DF features shows that the DF features correspond to the zero-order FC bright basis states. Each feature represents a group of levels which share the character of a zero-order FC bright level. Above 14 000 cm−1, characteristic groups of DF features with a width of around 300 cm−1 appear in the DF spectra originating from both v3=2 and v′3=3. The relative intensity patterns within each group of features in the two DF spectra are nearly identical. Three anharmonic resonances, including the DD resonance, are proposed as a plausible mechanism which splits a single FC bright state into several DF features. The SEP measurement revealed that a single DF feature splits further into several features with widths around 0.5 cm−1. The characteristic nested level structure identified in the DF and SEP spectra are explained in terms of a stepwise energy flow via a series of anharmonic resonances from the initially excited CC stretch/trans-bend vibrations to the remaining vibrational modes.

Quasistable extreme motion vibrational states of HFCO above its dissociation threshold

The anharmonic mixing of highly excited vibrational states of HFCO above its dissociation threshold is studied by stimulated emission pumping spectroscopy. At 0.05-cm−1 resolution, individual molecular eigenstates were resolved and state mixings observed by the distribution of oscillator strength to nearby dark states of the background. Most of the zero-order vibrational levels observed in the energy range between 13 000 and 23 000 cm−1 are assigned to long Franck–Condon-allowed progressions of extremely high overtones of the C–H out-of-plane bending mode (ν6) in combination with the C■O stretching mode (ν2). The extent of state mixing of highly excited vibrational states is strongly mode dependent. For vibrational states with almost the same total vibrational energy, states with the most quanta in ν6 show the least coupling with other states. More strikingly, as the total energy is increased by adding ν6 quanta, the state mixing becomes weaker. For the vibrational states observed above 18 000 cm−1, those that have all of the energy in ν6 or at most one quantum of excitation in ν2 are extremely stable against state mixing. These appear to be the regular states or quasiperiodic trajectories predicted in some theoretical studies of two-dimensional systems. Extreme motion of the C–H out-of-plane bending mode seems to localize and prevent coupling with other modes.

Collisional relaxation of CH 3O(Ã 2A 1, v3 = 1, K = 9, N = 9, J = 8.5) by He

The collision-induced relaxation of CH 3O in a selected single rotation—vibration level in the à 2A 1 first excited electronic state by He has been investigated using the technique of time-delayed stimulated-emission-pumping spectroscopy (SEP). CH 3O radicals were generated in a discharge flow reactor via the reaction of F atoms with CH 3OH. The target level was populated using a pulsed dye laser via the pump transition à 2A 1 ← X̃ 2E, 3 1 0, rR 21 (7.5, −8). The subsequent state-specific relaxation was monitored by observing the SEP signal induced by a second dye laser (dump) which was fired after a variable time delay with respect to the pump. The strength of the SEP signal at a given delay time directly reflects the population of the pumped à level. The rate constant (velocity-averaged cross section,±2σ) for overall collisional depopulation of the pumped level, CH 3O(Ã, υ 3 = 1, K = 9, N = 9, J = 8.5), by He and the collision-free lifetime of this level were found to be k = (1.75 ± 0.06) × 10 14 cm 3/mol s (21.8 Å 2) and τ = (1.8 ± 0.5) × 10 −6 s.

Stimulated-emission pumping for state-selective initiation of unimolecular reactions

Stimulated-emission pumping is successfully applied to the study of unimolecular dissociation dynamics of HFCO → HF + CO in a pulsed jet. Dissociative levels of the ground electronic state are prepared by stimulated-emission pumping, and the CO fragments formed from these levels are probed by vacuum-ultraviolet laser-induced fluorescence spectroscopy. The rotational energy distribution of the CO fragments indicates modest differences between the dissociation dynamics of HFCO and those of H2CO. However, the Doppler width of CO fragments from HFCO indicates that the total kinetic energy of the fragments from HFCO is ~65% of total available energy, the same fraction as in the dissociation of H2CO. The linewidths of the dissociative levels are also measured with high-resolution fluorescence-dip-detected stimulated-emission pumping spectroscopy.

Molecular dynamics and quantum chaos in small polyatomic molecules (CS_2, C_2H_2) through stimulated-emission pumping experiments and statistical Fourier-transform analysis

The experimental study of the quantum properties of polyatomic molecules at high vibrational energy when classical chaos is present (quantum chaos) is discussed. Such a study requires the improvement of both experimental and analytical techniques. Methods by which stimulated-emission pumping spectroscopy and statistical Fourier-transform analysis of spectra can be successfully used to achieve this goal are emphasized. Stimulated-emission pumping and the statistical Fourier transform share the same concept of wave-packet preparation and evolution. This dynamical approach leads to interesting observations in molecular spectra. A numerical model of two coupled Morse oscillators provides information about the physical significance of the correlation hole observed in the Fourier transform of spectra of chaotic molecules. A relevant example in the experimental spectrum of CS2 shows the destabilization of given periodic orbits in a regime of soft chaos. In such a case the destabilizing process starts with Fermi resonances. In CS2 the chaos appears first in a portion of the degree of freedom (symmetric stretching and bending). In C2H2, in contrast, a new periodic orbit appears, corresponding to a new low-frequency mode that emerges from the chaotic bath of C2H2 and that could induce specific chemical reactions.