Stimulated Emission Pumping Spectra Research Articles

The lowest lying excited electronic states for HFCO including a potential energy surface for S 1 in sum-of-products form

The vertical excitation energies to the S and T states for HFCO were determined using multi-reference (CASSCF, CASPT2, CASPT2-F12, MRCI, and MRCI-F12) and the single reference EOM-CCSD methods. The geometries, relative energies, and harmonic frequencies for the stationary points (minimum and transition state to inversion) on both the S and T potential energy surfaces (PESs) were characterised. A six-dimensional PES for the S state was constructed, in sum-of-products form using a neural network with exponential neurons, based on EOM-CCSD/aug-cc-pVTZ ab initio energies. Using the PES, the fundamental vibrational frequencies were computed using block-improved relaxation in MCTDH, and they agreed well with previous experiments (RMSE cm). A corresponding transition dipole moment surface (TDMS) was also fit to sum-of-products form. With the present PES and TDMS, along with the previous S surface, UV-vis absorption, stimulated emission pumping spectra, or optimal control of quantum dynamics via S can be explored.

Measuring Intermolecular Binding Energies by Laser Spectroscopy.

The ground-state dissociation energy, D0(S0), of isolated intermolecular complexes in the gas phase is a fundamental measure of the interaction strength between the molecules. We have developed a three-laser, triply resonant pump-dump-probe technique to measure dissociation energies of jet-cooled M•S complexes, where M is an aromatic chromophore and S is a closed-shell 'solvent' molecule. Stimulated emission pumping (SEP) via the S0→S1 electronic transition is used to precisely 'warm' the complex by populating high vibrational levels v" of the S0 state. If the deposited energy E(v") is less than D0(S0), the complex remains intact, and is then mass- and isomer-selectively detected by resonant two-photon ionization (R2PI) with a third (probe) laser. If the pumped level is above D0(S0), the hot complex dissociates and the probe signal disappears. Combining the fluorescence or SEP spectrum of the cold complex with the SEP breakoff of the hot complex brackets D0(S0). The UV chromophores 1-naphthol and carbazole were employed; these bind either dispersively via the aromatic rings, or form a hydrogen bond via the -OH or -NH group. Dissociation energies have been measured for dispersively bound complexes with noble gases (Ne, Kr, Ar, Xe), diatomics (N2, CO), alkanes (methane to n-butane), cycloalkanes (cyclopropane to cycloheptane), and unsaturated compounds (ethene, benzene). Hydrogen-bond dissociation energies have been measured for H2O, D2O, methanol, ethanol, ethers (oxirane, oxetane), NH3 and ND3.

Direct-potential-fit analyses yield improved empirical potentials for the ground $X\,^1\Sigma _g^+$XΣg+1 state of Be2

We have performed new direct-potential-fit (DPF) analyses of the rotationally resolved A (1)Π(u)(ν'=2,3;J' =1,2)→X(1)Σ(+)(g) (ν" ∈[0,11];J" ∈[0,3]) stimulated emission pumping spectra of Be2 [J. M. Merritt, V. E. Bondybey, and M. C. Heaven, Science 324, 1548 (2009)] using two quite different analytical potential energy functions that incorporate the correct theoretically known long-range behaviour in different ways. These functions are: the damped Morse/long-range potential [R. J. Le Roy, C. C. Haugen, J. Tao, and H. Li, Mol. Phys. 109, 435 (2011)], and the Chebyshev polynomial expansion potential [L. Busevica, I. Klincare, O. Nikolayeva, M. Tamanis, R. Ferber, V. V. Meshkov, E. A. Pazyuk, and A. V. Stolyarov, J. Chem. Phys. 134, 104307 (2011)]. In contrast with the expanded Morse oscillator potential determined in the original DPF analysis of Merritt et al. [Science 324, 1548 (2009)], both of these functions unambiguously support the existence of the v″ = 11 last vibrational levels which is bound by only ∼0.5 cm(-1), and they give equivalent, essentially exact predictions for this level when using the original data set which ended at v″ = 10. These empirical potentials predict an equilibrium distance of re = 2.445(5) Å and a well depth of D(e) = 934.9(0.4) cm(-1), values which agree (within the uncertainties) with the best ab initio estimates of 2.444(10) Å and 935(10) cm(-1), respectively [J. Koput, Phys. Chem. Chem. Phys. 13, 20311 (2011)].

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.

ChemInform Abstract: 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 A 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 X 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 A 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 un...

The spectroscopic characterization of the methoxy radical. II. Rotationally resolved à A21-X̃ E2 electronic and X̃ E2 microwave spectra of the perdeuteromethoxy radical CD3O

Rotationally resolved laser-induced fluorescence (LIF) and stimulated emission pumping (SEP) A (2)A(1)-X (2)E spectra of the perdeuteromethoxy radical (CD(3)O) have been observed. These data directly connect the two spin-orbit components (E(1/2) and E(3/2)) of the ground electronic state with high precision. Molecular constants for both electronic states are determined in a global fitting that involves LIF, SEP, and pure rotational spectra in the microwave region. For the microwave transitions, the resolved hyperfine structure is analyzed providing molecular parameters characterizing it and hyperfine-free transitions for the global fitting. A complete "experimental" geometry for the methoxy radical at the C(3v) conical intersection is determined from the rotational constants of its isotopologs. The experimental isotopic dependence of other parameters in the effective Hamiltonians is compared to the theoretically expected variation. These comparisons allow considerable insight into the physical significance of a number of parameters in the effective Hamiltonian. In particular, experimental evidence is found for a previously predicted vibrational correction to the A rotational constant of a Jahn-Teller active molecule.

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.

Variational Calculation of Specific, Highly Excited Vibrational States in DFCO: Comparison with Experimental Data

A stimulated emission pumping spectra of jet-cooled DFCO performed by Crane et al. (J. Mol. Spectrosc. 1997, 183, 273) has provided a great number of ro-vibrational lines up to 9000 cm(-1) of excitation energy. By combining a Jacobi-Wilson (JW) approach with a Davidson scheme, we calculate the lines provided by the experiment up to 9000 cm(-1) using an ab initio global potential energy surface (PES) developed by Kato et al. (J. Chem. Phys. 1997, 107, 6114). Comparisons between experimental and calculated data provide a critical test of the quality of the PES used. We show that the variational calculated energies can be efficiently corrected by taking into account the error observed for the A' fundamental transitions nu(i) (i = 1, ..., 5) and the first overtone 2nu(6). A detailed analysis of the eigenstates obtained by the calculation allows one to quantify the coupling between the different modes. Such an information is essential to understand and predict the energy flow through a DFCO molecule that is initially excited.

THEORETICAL STUDIES OF $\tilde{A}{}^1 A^{\prime\prime}\to\tilde{X}{}^1 A^\prime$ RESONANCE EMISSION SPECTRA OF HCN/DCN USING SINGLE LANCZOS PROPAGATION METHOD

Using an efficient single Lanczos propagation method, we report the [Formula: see text] resonance emission spectra of HCN and DCN from a number of low-lying vibrational levels of the Ã-state manifold. Our calculations represent the first such undertaking in which a high-quality ab initio based potential energy surface of the excited (Ã1 A″) state and a [Formula: see text] transition dipole surface were used. The results show a significant improvement over previous theoretical work in reproducing experimental stimulated emission pumping spectra of HCN. The improved theory-experiment agreement is attributed to the accurate Ã-state potential energy surface, while the impact of the transition dipole function was found to be relatively minor.

Discovery of the optically forbidden S1–S transition of silylidene (H2C=Si)

The electronically forbidden à 1A2–X̃ 1A1 band system of jet-cooled silylidene (H2CSi and D2CSi) has been detected for the first time using laser-induced fluorescence (LIF) and stimulated emission pumping (SEP) techniques. The very weak, vibronically induced 401 and 301401 bands were detected by LIF along with the corresponding 601 and 301601 bands which gain intensity through excited state Coriolis coupling. SEP spectra, obtained by pumping the 000 band of the S2 state and stimulating transitions down to the S1 state through the allowed S2–S1 transition, revealed many more bands, including the 000 bands, which were studied at high resolution and rotationally analyzed. From the upper state rotational constants of H2CSi and D2CSi, the excited state structure was obtained as r0′ (SiC)=1.873(2) Å, r0′(CH)=1.099(5) Å, and θ0′(HCH)=113.9(3)°. The four lowest energy excited state vibrational frequencies of both isotopomers have also been determined. High level ab initio predictions of the ground and excited state properties of silylidene are also reported and found to be in good agreement with the experimentally determined values.

Intramolecular vibrational energy redistribution, mode specificity, and nonexponential unimolecular decay dynamics of vibrationally highly excited states of DCO (X̃ 2A′)

The unimolecular dynamics of vibrationally highly excited states of DCO (X̃ 2A′) in the energy region up to Evib⩽9500 cm−1, beyond the D–CO (X̃) dissociation threshold, has been investigated using an effective polyad Hamiltonian obtained by fitting to the term energies from the measured B̃ 2A′←X̃ 2A′ stimulated emission pumping (SEP) spectra of the molecule [Stöck et al., J. Chem. Phys. 106, 5333 (1997); Temps and Tröllsch, Z. Phys. Chem. 215, 207 (2001)]. An added absorbing negative imaginary potential allowed for the unimolecular dissociation of the highly excited DCO via distinctive open reaction channels of the DC stretching vibration. The ensuing dynamics was explored using a wave packet propagation approach. Time profiles describing the intramolecular vibrational energy redistribution (IVR) and unimolecular decay kinetics were computed for the CO stretching zero-order basis states up to 6 quanta of excitation and the DCO bending zero-order basis states up to 12 quanta of excitation. The computed decay curves for the CO stretching zero-order basis states compare nicely with those of the respective coherent superposition states constructed directly from the measured SEP spectra (assuming the CO stretching mode as the Franck–Condon active bright zero-order mode that determines the observed transitions). A comparison of the decay curves with those of the almost isoenergetic DCO bending zero-order basis states in the respective polyads reveals large differences in the couplings of the two vibrational modes among each other and with the open dissociation channels. The obtained unimolecular decay profiles exhibit pronounced non-exponential kinetics. Comparison with statistically calculated decay rates shows a substantial degree of mode specificity of the dynamics, which can be attributed to a bottleneck in the IVR from the CO stretching vibration to the reaction coordinate. The model calculations explain the two-to-three orders of magnitude large difference between the measured eigenstate specific DCO (X̃) decay constants [Stöck et al.] and predictions by microcanonical statistical rate theories.

Stimulated Emission Pumping Spectroscopy of SiH 2: First Observation of the Spin–Orbit Interaction between the X̃1A1 and the ã3B1 States

Stimulated emission pumping spectra of the à 1 B 1– X̃ 1 A 1 transition of the SiH 2 radical were observed in order to obtain information about the ã 3 B 1 state through the spin–orbit interaction. The vibrational level structure of the X̃ state, which is the basis for the present observation of the triplet state, was well described with a polyad structure, in which both the 1ν 1 : 2ν 2 Fermi and the 2ν 1 : 2ν 3 Darling–Dennison anharmonic resonances were considered. In the P=10 polyad, four sets of spin–orbit perturbations were observed for the first time. The triplet state observed at about 9640 cm −1 from the (000) level of the X̃ state was tentatively assigned as the ã 3 B 1 (030) level. An analysis of the spin–orbit interaction showed that the interaction energies of the spin–orbit coupling are 0.73–3.13 cm −1. This value is rather smaller than that expected based on the comparison with CH 2. This is considered to be due to poor overlap between the vibrational wave functions in the ã and the X̃ state.

Near-Infrared Diode Laser Spectroscopy of FeC in the 0.8-μm Region: A Simultaneous Analysis of the X3Δi and [3.8]1Δ States

The near-infrared absorption spectra of the FeC radical have been measured by diode laser spectroscopy in the regions from 11 810 to 13 200 cm−1. Rotationally resolved 28 new vibronic bands of the 56Fe12C major isotopic species were observed as well as two known bands. Five of them were assigned as the v′=v″=0–4 sequence bands in the [13.1]3Φ4←X3Δ3 system, six are the v′=v″=0–5 sequences in the [13.18]Ω=3←X3Δ2 system, and five were assigned as the v′=v″=0–4 sequences in the [13.5]Ω=2←X3Δ1 system. The X3Δ1 state of FeC is reported for the first time but we found that the rotational constant for this state of 56FeC is very close to that for the X3Δ3 state of 54FeC. Transitions of the 54FeC minor isotopic species for the two known bands were thus reinvestigated. For the unclassified 11 new bands, the lower states were assigned as the v=0–3 states in X3Δi. The energy difference between the Ω=2 and 3 sublevels in the X3Δi state could be determined to be 329.8059±0.0005 cm−1 as a results of the analysis of the two of new bands, [12.32]←X3Δ3v=0 and [12.32]←X3Δ2v=0 observed near 12 317 cm−1 and 11 987 cm−1, respectively. Stimulated emission pumping spectra were also recorded near 17 600 cm−1 to examine the low-lying electronic state which had been observed 3460 cm−1 above the X3Δ2 state (K. Aiuchi and K. Tsuji and K. Shibuya, Chem. Phys. Lett.309, 229–233 (1999)), and it was assigned as the [3.8]1Δ state arising from the same configuration as that of X3Δi (…3π41δ39σ1). In addition, the v=0–0 and 1–1 bands of the [16.2]Ω=3←[3.8]1Δ system were observed by near-infrared diode laser spectroscopy. The lower state combination differences were calculated for the possible sets of the energy levels in the X3Δi and [3.8]1Δ states with v=0–5 by using the present result and the other spectroscopic data reported. A simultaneous analysis was carried out by using the combination differences to determine the molecular constants for the X3Δi and [3.8]1Δ states as well as the off-diagonal spin–orbit interaction terms. The [13.18]Ω=3 state and the [13.5]Ω=2 state were concluded to be other two spin sublevels for the [13.1]3Φ4 state and the [16.2]Ω=3 state was assigned as an isoconfigurational 1Φ state.

The ground state of silylidene (H2C=Si), the silicon analog of vinylidene, from stimulated emission pumping and wavelength-resolved fluorescence spectroscopy

The ground state vibrational energy levels of jet-cooled H2CSi and D2CSi have been studied by a combination of wavelength-resolved fluorescence and stimulated emission pumping (SEP) techniques. By taking advantage of the vibrational selection rules and Franck–Condon factors and selectively pumping upper state single vibronic levels, readily assignable low-resolution emission spectra were obtained. Higher resolution SEP spectra were recorded to give precise measurements of the vibrational band origins of many of the lower-lying vibrational levels. All of the vibrational frequencies, except for the Franck–Condon inactive CH asymmetric stretching mode, ν5, have been determined for both isotopomers. The CH2 rocking mode (ν6) is found to have a very low 263 cm−1 frequency in the ground state. The spectra are complicated by unexpected activity in the out-of-plane bending vibration.

Exploring the transition state for the Li+HF→LiF+H reaction through the A←X absorption spectrum and X←A stimulated emission pumping

The A(2 2A′) first excited electronic state of LiHF has been calculated (about 3300 points at MRDCI level) and an analytical fit of the global potential energy surface is presented. The A←X absorption spectrum is simulated at 10 K between 9000 and 13 000 cm−1, what involves the calculation of excited bound states for moderately high total angular momentum. These quasibound levels of the A state can only decay by electronic predissociation (EP) towards the X ground state, since the spontaneous radiative emission is considered to be very slow. The decay of such states has been estimated using a perturbative approach and it is found that LiF products are produced with a high efficiency (>80%). The X←A stimulated emission pumping spectra is simulated for several initial quasibound levels. These spectra allow the examination of the reaction dynamics specially near the transition state region, and the first photon excitation may act as a rotational filter, reducing the problem of the partial wave average involved in reactive collisions. The high reaction efficiency of this last process is also discussed.

Vibrational spectroscopy of phosphaethyne (HCP). I. Potential energy surface, variational calculations, and comparison with experimental data

A new potential energy surface for the electronic ground state of HCP (phosphaethyne) is presented. The ab initio calculations are based on the internally contracted multireference configuration interaction method using atomic basis functions of quintuple-zeta quality. The ca. 1 000 calculated energy points are fitted to a complex analytical function, which is employed in the subsequent quantum-mechanical variational calculations for total angular momentum J=0–2. The majority of the first 850 vibrational states is assigned in terms of three quantum numbers. The calculated energies are compared to various sets of experimental data—obtained from high-resolution Fourier-transform infrared spectra, dispersed fluorescence spectra, and stimulated-emission pumping spectra. The energy regime, which is covered, extends up to about 25 000 cm−1 above the ground vibrational state. The agreement is excellent; every experimentally assigned level is uniquely related to a calculated vibrational state. Some experimental misassignments at the lower ends of the high-energy polyads are corrected. The progression of “isomerization” (i.e., large-amplitude bending) states, which was experimentally observed by Ishikawa et al. [J. Chem. Phys. 106, 2980 (1997)], is quantitatively confirmed.

Fermi-resonances, potential energy function, and cluster-like spectra of the ground electronic state of CS 2

The rotationally resolved stimulated emission pumping spectra of the ground electronic state X̌ 1Σ + g of CS 2 from 3290 to 16 370 cm −1 have been recorded by using a three-level polarization spectroscopy technique. The vibrational analysis was carried out by using a normal coordinate model. In addition to the 〈 ν 1−1, ν 2 ℓ+2, ν 3|H| ν 1, ν 2 ℓ, ν 3〉 Fermi-resonance, the 〈 ν 1−3, ν 2 ℓ+4, ν 3|H| ν 1, ν 2 ℓ, ν 3〉 type of anharmonic resonance was found. The parameters of the potential energy function were given in both the normal mode and the internal coordinate representations. As the energy was increased to above 13 400 cm −1, the spectra gradually developed cluster-like features. Such cluster-like features were interpreted as anharmonic resonance polyads. The dependence of the onset of the cluster-like features on the energy and on the initial conditions was studied by using a classical trajectory calculation.

Theoretical studies of the potential energy surface and wavepacket dynamics of the Li 3 system

The three-dimensional (3D) potential energy surface of the ground state of Li3 was determined by the multireference configuration interaction method. The vibrational motions and pseudorotation were investigated by a 3D time-dependent wavepacket formalism. The analytical expression of the 3D surface is given and the results of vibrational analyses at several critical points are presented. The low-lying excited states of Li3 were examined for the C 2 v structure and the vertical and adiabatic excitation energies were calculated. The ground and singlet excited states of Li2 were calculated and their spectroscopic constants compare well with the experimental values. A 3D wavepacket calculation was performed for simulations of the stimulated emission pumping spectrum in which the A state was taken as an intermediate. The recurrences of the autocorrelation functions were characterized by classical trajectory calculations. The autocorrelation functions obtained by wavepacket propagation are reproduced well by the accumulation of the classical trajectories in the short-time region.

The ground state vibrational structure of SCCl 2: : observation of backbone IVR

Vibrationally excited SCCl 2 is a model system for energy redistribution among backbone vibrational modes. Over 200 transitions in the X̃←B̃ dispersed fluorescence spectrum of thiophosgene have been observed to map the ground electronic surface up to 15000 cm −1. An analytical six-dimensional potential surface derived from ab initio calculations has been fitted directly to about 100 of the transitions to provide an experimentally derived surface for intramolecular vibrational energy redistribution (IVR) quantum dynamics calculations. Higher resolution stimulated emission pumping spectra from different vibrational levels in the B̃ state reveal a complex progression from unfragmented/slightly Fermi-resonant states, to highly fragmented states above v 1=8.

Vibrational Assignment and Anharmonic Resonance Analysis of the Dispersed Fluorescence and Stimulated Emission Pumping Spectra of DFCO (S0) up to 9000 cm−1

Dispersed fluorescence and stimulated emission pumping spectra of jet-cooled DFCO from the 2162, 5164, 215162, and 5166vibrational states ofS1are presented up to 9000 cm−1. Progressions are assigned primarily to vibrational states with excitation in the Franck–Condon active modes ν2, ν5, and ν6. The spectrum is perturbed by a 266 (ν2≈ 2ν6), a 233 (ν2≈ 2ν3), and a 3566 (ν3+ ν5≈ 2ν6) anharmonic resonance. These three resonances are increasingly important at high vibrational energies, thus explaining the complexity of the DFCO spectrum compared to that of HFCO. Seventy-five vibrational states are assigned as eigenstates of a 266, 233, and 3566 anharmonic resonance Hamiltonian. The rms deviation in the calculated energies is 1.2 cm−1. Harmonic frequencies and anharmonic constants are reported. These are compared to constants derived fromab initiopotential energy surfaces.