Real-time observation of multi-mode vibronic coherence in pentafluoropyridine.

The ultrafast dynamics of pentafluoropyridine in the 1 1B2 (ππ*) electronic state excited at λpump = 255 nm is investigated by femtosecond time-resolved time-of-flight mass spectrometry and photoelectron imaging spectroscopy. A pronounced, long-lived, and complex periodic modulation of the transient ion yield signal with contributions by four distinct frequency components, 72 cm-1, 144 cm-1, 251 cm-1, and 281 cm-1, is observed for up to 9 ps. The recorded photoelectron images display a spectral band from the excited 1 1B2 (ππ*) state only in the oscillation maxima; the signal is strongly reduced in the oscillation minima. Supported by electronic structure calculations at the RI-SCS-CC2 and XMCQDPT2 levels of theory, the oscillating components of the signal are identified as frequencies of b1 symmetry coupling modes in a vibronic coherence of the 1 1B2 (ππ*) and 1 1A2 (πσ*) electronic states. The optical excitation initiates regular and periodic wavepacket motion along those out-of-plane modes. In the distorted molecular structure, the initially excited state acquires substantial πσ* character that modulates the transition dipole moment for ionization and results in the observed oscillations.

Note: Energy calibration of a femtosecond photoelectron imaging detector with correction for the ponderomotive shift of atomic ionization energies.

Femtosecond photoelectron imaging spectroscopy is a powerful technique for following state-resolved molecular transformations in complex coupled potential energy landscapes. To avoid unwanted nonlinear side-effects, the employed laser pulse energies are usually reduced to minimal values. However, the energy calibration of the photoelectron imaging detector is ideally performed using multi-photon above-threshold ionization of suitable atomic species, for which rather high laser intensities are required. In this work, we show that the calibration spectra of xenon obtained with high laser pulse energies cannot be directly used for the evaluation of molecular photoelectron spectra recorded using low-energy laser pulses. The reason is the intensity-dependent AC Stark shift of the atomic ionization energies to larger values, which in turn leads to a corresponding decrease of the photoelectron kinetic energies. We present a simple procedure to quantify this so-called ponderomotive shift and calculate the theoretically expected un-shifted photoelectron energies.

Long-lived coherence in pentafluorobenzene as a probe of ππ(*) - πσ(*) vibronic coupling.

The dynamics of pentafluorobenzene after femtosecond laser excitation to the optically bright ππ(*) first excited electronic state have been investigated by femtosecond time-resolved time-of-flight mass spectrometry and femtosecond time-resolved photoelectron imaging spectroscopy. The observed temporal profiles exhibit a bi-exponential decay behavior with a superimposed, long-lived, large-amplitude oscillation with a frequency of νosc = 78-74 cm(-1) and a damping time of τD = 5-2 ps. On the basis of electronic structure and quantum dynamics calculations, the oscillations have been shown to arise due to vibronic coupling between the optically bright ππ(*) state and the energetically close-lying optically dark πσ(*) state. The coupling leads to a pronounced double-well character of the lowest excited adiabatic potential energy surface along several out-of-plane modes of b1 symmetry. The optical electronic excitation initiates periodic wavepacket motion along these modes. In the out-of-plane distorted molecular configuration, the excited state acquires substantial πσ(*) character, thus modulating the ionization probability. The photoelectron spectra and the anisotropy of their angular distribution confirm the periodically changing electronic character. The ionizing probe laser pulse directly maps the coupled electron-nuclear motion into the observed signal oscillations.

Ultrafast α -CC bond cleavage of acetone upon excitation to 3p and 3d Rydberg states by femtosecond time-resolved photoelectron imaging.

The radiationless electronic relaxation and α -CC bond fission dynamics of jet-cooled acetone in the S1 (nπ*) state and in high-lying 3p and 3d Rydberg states have been investigated by femtosecond time-resolved mass spectrometry and photoelectron imaging. The S1 state was accessed by absorption of a UV pump photon at selected wavelengths between λ = 320 and 250 nm. The observed acetone mass signals and the S1 photoelectron band decayed on sub-picosecond time scales, consistent with a recently proposed ultrafast structural relaxation of the molecules in the S1 state away from the Franck-Condon probe window. No direct signatures could be observed by the experiments for CC dissociation on the S1 potential energy hypersurface in up to 1 ns. The observed acetyl mass signals at all pump wavelengths turned out to be associated with absorption by the molecules of one or more additional pump and/or probe photons. In particular, absorption of a second UV pump photon by the S1 (nπ*) state was found to populate a series of high-lying states belonging to the n = 3 Rydberg manifold. The respective transitions are favored by much larger cross sections compared to the S1 ← S0 transition. The characteristic energies revealed by the photoelectron images allowed for assignments to the 3p and 3dyz states. At two-photon excitation energies higher than 8.1 eV, an ultrafast reaction pathway for breaking the α -CC bond in 50-90 fs via the 3dyz Rydberg state and the elusive ππ* state was observed, explaining the formation of acetyl radicals after femtosecond laser excitation of acetone at these wavelengths.

Validation of the extended simultaneous kinetics and ringdown model by measurements of the reaction NH2 + NO.

The determination of rate constants for fast chemical reactions from nonexponential cavity ringdown profiles requires a consideration of the interfering laser bandwidth effect that arises if the line width of the ringdown probe laser exceeds the absorption line width of the detected species. The deconvolution of the kinetics and the bandwidth effect can be accomplished with the extended simultaneous kinetics and ringdown (eSKaR) model presented by Guo et al. (Guo, et al. Phys. Chem. Chem. Phys. 2003, 5, 4622). We present a detailed validation of this eSKaR model by a corresponding investigation of the well-known rate constant for the reaction NH2 + NO. Line profiles of the pulsed ringdown probe laser and the NH2 absorption line were determined from forward modeling of experimental ringdown profiles and verified by narrow-bandwidth laser absorption measurements. In addition, the rate constant for the title reaction was evaluated using the eSKaR model and also by means of a conventional pump-probe approach with variable time delays between the photolysis (pump) and ringdown (probe) laser pulses. The resulting room temperature rate constant for the NH2 + NO reaction, k1= (8.5 +/- 1.0) x 10(12) cm(3) mol(-1) s(-1), and the room temperature pressure broadening coefficient of NH2, = 2.27 GHz/bar, measured on the A2A1<-- X2B1 transition at wavelengths around lambda = 597 nm, were found to be in excellent agreement with the available literature data.

Femtosecond fluorescence up-conversion spectroscopy of a rotation-restricted azobenzene after excitation to the S1 state.

Femtosecond time-resolved fluorescence up-conversion spectroscopy has been used in a study of the photoinduced isomerization reactions of a rotation-restricted trans-azobenzene (trans-AB) derivative capped by a crown ether (1), a chemically similar open derivative (2), and unsubstituted trans-AB (3) after excitation to the S1 (npi*) state at lamda=475 nm in dioxane solution. The observed biexponential temporal fluorescence profiles for 1 and 2 were almost indistinguishable within experimental error. The fitted fast fluorescence decay times (+/-2sigma) for the two compounds were tau1 (1) = (0.79 +/- 0.20) and tau1 (2) = (1.05 +/- 0.20) ps, compared to tau1 (3) = (0.37 +/- 0.06) ps. The second decay components could be described with tau2 (1) = (20.3 +/- 9.5) resp. tau2 (2) = (19.0 +/- 6.0) ps, vs. tau2 (3) = (3.26 +/- 0.85) ps. The very similar lifetimes strongly suggest that trans-cis isomerization of 1 and 2 after S1 excitation is governed by the same mechanism. Since 1 cannot isomerize by a simple large-amplitude rotation of one of the phenyl rings about the central NN bond, the isomerization dynamics of both ABs should be better described as "inversion" at the N atom(s) rather than large-amplitude "rotation".

Photodissociation dynamics of pyrrole: evidence for mode specific dynamics from conical intersections.

The H and D atom elimination mechanisms in the photodissociation of jet cooled pyrrole and pyrrole-d1 have been studied by photofragment velocity map imaging. The molecules were excited to the 1 1A2 (pi sigma*) state at lambda = 243 nm and to the 1 1B2 (pi pi*) state at lambda = 217 nm. H/D atoms were detected by (2 + 1) resonance enhanced multiphoton ionization (REMPI) at lambda = 243 nm. The analysis of the images and the resulting translational energy distributions from the 1 1A2 state demonstrates the existence of two decay pathways, fast mode-specific cleavage of the NH bond in the excited state (channel A) and internal conversion (IC) to the electronic ground state (S0) followed by unimolecular decomposition of the vibrationally hot S0 molecules (channel B). The angular distributions of the H/D atoms from the direct dissociation in the excited state are strongly anisotropic, whereas the decay of the S0 molecules leads to spatially isotropic distributions. The results at lambda = 217 nm indicate that the 1 1B2 state undergoes an ultrafast radiationless transition to 1 1A2 followed by the abovementioned direct mode-specific NH bond fission on the 1 1A2 potential energy surface (channel A') or conversion to S0 and subsequent unimolecular decomposition (channel B'). The latter pathway may also be initiated by a direct relaxation from 1 1B2 to S0. The anisotropy parameter of beta approximately -1 for the direct NH bond fission at lambda = 217 nm is in accordance with the expectations for a perpendicular electronic excitation and a dissociation lifetime that is short compared to the rotational period of the molecules. The fast decay dynamics of both excited electronic states can be rationalized with reference to the theoretically predicted conical intersections between the pi pi*, pi sigma*, and S0 potential energy surfaces and the antibonding nature of the pi sigma* potential energy surface with respect to the NH bond [A. L. Sobolewski, W. Domcke. C. Dedonder-Lardeux and C. Jouvet, Phys. Chem. Chem. Phys. 2002, 4, 1093].

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.

An experimental and theoretical study of the product distribution of the reaction CH2 (X 3B1) + NO.

Measurements of the product branching ratios of the reaction CH2 (X 3B1) + NO (1) are presented together with calculations of the thermal rate constant and branching ratios using unimolecular rate theory. The reaction was investigated experimentally at room temperature using FTIR spectroscopy. The yields of the main products HCNO and HCN were found to be gamma HCNO = 0.89 +/- 0.06, gamma HCN = 0.11 +/- 0.06. Other minor products could be rationalized by numerical simulations of the reaction system taking into account possible consecutive reactions. The potential energy surface for the reaction was characterized by quantum chemical calculations using ab initio and density functional methods. The proposed reaction pathways connecting reactants to products were explored by multi-channel unimolecular rate theory calculations to determine the CH2 (X) + NO capture rate constant and the rate constants for the different product channels as a function of temperature. The calculated capture rate constant of k = 2.3 x 10(13) cm3 mol-1 s-1 is in good agreement with experimental values at room temperature. Collisional stabilization of the initial H2CNO recombination complex was predicted to be negligible up to pressures of > 1 bar. For ambient pressures and temperatures up to 2000 K, HCNO + H were calculated as the dominating products, with gamma HCNO approximately 0.94 in agreement with the experiments. The channel to HCN + OH was calculated with 0.015 < or = gamma HCN < or = 0.05, only slightly below the experimental value.

The Far-Infrared Laser Magnetic Resonance Spectrum of CH2F.

Far-infrared laser magnetic resonance (FIR-LMR) spectra due to the CH2F radical have been recorded on seven laser lines at wavelengths between 301 and 568 µm. Observed resonances were assigned to fine and hyperfine components of pure rotational transitions of CH2F in the ground vibrational state and the first excited state of the nu4 out-of-plane bending mode. All assigned transitions obey a-dipole selection rules. The data were combined with previously reported microwave results (Y. Endo, C. Yamada, S. Saito, and E. Hirota, J. Chem. Phys. 79, 1605 (1983)) and subjected to a least-squares fit to determine the parameters of the effective Hamiltonian describing the v4 = 0 and 1 vibrational levels of the CH2F radical. Copyright 1999 Academic Press.