Georg Seyfang

Infrared multiphoton excitation, dissociation and ionization of C60

Abstract We report the observation of mid-infrared multiphoton excitation, dissociation, and ionization of C 60 by shaped CO 2 laser pulses. The results are interpreted in the framework of the general statistical theory of infrared laser chemistry, adding some simple model assumptions. The observations on intensity- and fluence-dependent fragment ion distributions are consistent with a mechanism of vibrational preionization from energies exceeding the threshold for ionization by about a factor of 8.

Time‐resolved infrared‐spectroscopic observation of relaxation and reaction processes during and after infrared‐multiphoton excitation of 12CF3I and 13CF3I with shaped nanosecond pulses

We have produced shaped infrared laser pulses of several kinds ranging from about 2–100 ns duration using a line tuned CO2 laser combined with intracavity absorbers and a CdTe electro‐optical switch. The time‐dependent infrared absorption of 12CF3I and 13CF3I during and after infrared‐multiphoton excitation with these pulses was followed by means of a line tuned continuous wave‐CO2 laser and a fast HgCdTe infrared detector (time resolution about 1 ns). The effective time‐dependent absorption cross section shows fluence‐dependent decay at large fluence with an effective exponential decay constant kI,σ≂1.12 cm2 J−1. This can be interpreted by first generation and then decay by further radiative pumping of highly excited levels of CF3I. The results have been analyzed by master equation modeling using a nonlinear case B/C master equation for multiphoton excitation and very simple models for the absorption properties of highly excited molecules. After nanosecond excitation to very high levels, one finds unimol...

Generation of shaped pulses for IR — laser chemistry

Abstract 10 μm laser pulses with different temporal shape and a duration between 2 and 80 ns were produced in a TEA-CO2 laser by the technique of a saturable absorber in an intracavity cell. At low absorber gas pressures (≤ 400 Pa) single longitudinal mode operation of the laser was obtained at a great number of CO2 laser lines with 9 different selective absorbers. Stable mode locked operation has been achieved over the entire range of the CO2 laser leading to pulses of 2 ns duration. Single peaks of the mode locked pulse train were sliced out by a fast CdTe electro-optical switch and amplified in a second CO2 laser up to an energy of 2 J corresponding to an intensity of 1 GW. The spectral properties of the useful absorbers are presented systematically. As an application we show some quantitative results on the nonlinear intensity dependence of the IR-laser chemical reaction CF3B→nhvCF3 + Br in the transition range from case C to case B.

Diode laser detection of iodine atom hyperfine transitions during and after infrared multiphoton excitation and dissociation of CF3I with short pulse CO2 lasers

Abstract The iodine atom concentration has been measured by time-resolve absorption spectroscopy for all hyperfine levels on the I(2P 3 2 )→I(2P 1 2 )) transition during and after the multiphoton excitation of CF3I, using a tunable diode laser in the near infrared at 1.3 μm. The experiments have been performed under collision free conditions with a time resolution of about 1 ns. Using CO2 laser pulses of different temporal shape, the nonlinear intensity dependence of the multiphoton excitation process and the previously measured steady state rate constants have been confirmed. Detection at different hyperfine transistion frequencies and charging the relative orientation of the polarization of the pump and probe laser showed statistical behaviour for the population of the hyperfine levels and for the orientation of the iodine atoms. Using CO2 laser pulses of specially tailored temporal shape the theoretically predicted intensity fall-off for the steady state rate constant has been found experimentally.

Infrared laser induced population transfer and parity selection in 14NH3: A proof of principle experiment towards detecting parity violation in chiral molecules

We have set up an experiment for the efficient population transfer by a sequential two photon-absorption and stimulated emission-process in a molecular beam to prepare quantum states of well defined parity and their subsequent sensitive detection. This provides a proof of principle for an experiment which would allow for parity selection and measurement of the time evolution of parity in chiral molecules, resulting in a measurement of the parity violating energy difference ΔpvE between enantiomers of chiral molecules. Here, we present first results on a simple achiral molecule demonstrating efficient population transfer (about 80% on the average for each step) and unperturbed persistence of a selected excited parity level over flight times of about 1.3 ms in the beam. In agreement with model calculations with and without including nuclear hyperfine structure, efficient population transfer can be achieved by a rather simple implementation of the rapid adiabatic passage method of Reuss and coworkers and considering also the stimulated Raman adiabatic passage technique of Bergmann and coworkers as an alternative. The preparation step uses two powerful single mode continuous wave optical parametric oscillators of high frequency stability and accuracy. The detection uses a sensitive resonantly enhanced multiphoton ionization method after free flight lengths of up to 0.8 m in the molecular beam. Using this technique, we were able to also resolve the nuclear hyperfine structure in the rovibrational levels of the ν1 and ν3 fundamentals as well as the 2ν4 overtone of (14)NH3, for which no previous data with hyperfine resolution were available. We present our new results on the quadrupole coupling constants for the ν1, ν3, and 2ν4 levels in the context of previously known data for ν2 and its overtone, as well as ν4, and the ground state. Thus, now, (14)N quadrupole coupling constants for all fundamentals and some overtones of (14)NH3 are known and can be used for further theoretical analysis.

Time-resolved detection of reaction products in the infrared laser chemistry of sulfoxides: C2H4SO, (CH3)2SO, (CD3)2SO

Abstract The first observations of the infrared laser chemical reaction of organic sulfoxides after IR multiphoton excitation of the SO infrared chromophore (with large band strength G =2.0 pm 2 in (CH 3 ) 2 SO) with pulsed CO 2 laser radiation are reported. The collisionless IR photochemical primary products are shown to be C 2 H 4 and SO ( 3 Σ − ) for C 2 H 4 SO, and CH 3 and ( 3 Σ − ) SO (via CH 3 SO) for (CH 3 ) 2 SO by means of time-resolved laser spectroscopic observation of C 2 H 4 , SO, and CD 3 . Effective laser chemical rate constants k I (st)=1.2 cm 2 J −1 for C 2 H 4 SO (9R32) and 0.06 cm 2 J −1 for (CH 3 ) 2 SO (9R32) are obtained from static yield measurements as well as unimolecular decay constants k ≈10 7 to 10 8 s −1 (for C 2 H 4 SO) after excitation above threshold and laser probing of C 2 H 4 and SO. A nonlinear intensity dependence of k (st) is reported for C 2 H 4 SO and nanosecond intramolecular relaxation for (CH 3 ) 2 SO.

Nonlinear intensity dependence in the infrared multiphoton excitation and dissociation of methanol pre-excited to different energies

We report quantitative dissociation yields for the reaction CH3OH (v(OH))-->nhnu CH3+OH induced by infrared multiphoton excitation of methanol pre-excited to various levels of the OH stretching vibration (v(OH)=0, 1, 3, 5). The yields are measured by detecting OH using laser induced fluorescence. It is demonstrated that for low levels of pre-excitation (v(OH)=0, 1, 3) there is a substantial nonlinear intensity dependence, as a higher yield is found for self mode-locked CO2 laser pulses (with higher peak intensity) as compared to single mode pulses of the same laser fluence, but lower peak intensity. In contrast, at high levels of preexcitation (v(OH)=5) this nonlinear intensity dependence is absent. Quantitative model calculations are carried out using a case B/case C master equation approach that takes nonlinear intensity dependence into account. The calculations are consistent with the experimental results and confirm the prediction that an important part of the selectivity of the CO2 laser excitation step in infrared laser assisted photofragment spectroscopy of CH3OH is due to this nonlinear intensity dependence. We discuss further consequences of these experimental observations and theoretical predictions, which are also extended to infrared multiphoton excitation of C2H5OH. Infrared (C-O) chromophore band strengths are reported for CH3OH and C2H5OH

Femtosecond intramolecular dymanics after near-IR excitation of CH3I, C2H5I, CF3CHFI, and C7H8 molecules in the gas phase and in solution

The rapid flow of vibrational energy within a molecule is central for the control and as well for the theory of unimolecular reactions. It defines the lifetime of vibrationally excited states and thus the time during which a specific vibrational excitation can control the outcome of chemical reactions. Times for intramolecular vibrational energy redistribution (IVR) can be deduced either indirectly from time-independent high resolution infrared(IR) spectra or measured directly in kinetic pump-probe experiments. We have applied delayed ultraviolet(UV) absorption spectroscopy with a time resolution of 150 fs to measure intramolecular vibrational energy redistribution after near-IR excitation of the CH-stretching vibration around 5900 cm-1 in CF3CHFI, CH3I, C2H5I, and C7H8. Intramolecular relaxation times T(IVR) between 3 and 7 ps have been found in the gas phase. For CH3I an additional short time of 250 fs has been measured. In the liquid phase IVR is followed by a fast collisional energy transfer of the excitation energy to the solvent molecules. Assuming a two step kinetic mechanism intermolecular relaxation times T(transfer) between 10 and 30 ps have been determined.

Dynamics of unimolecular reactions induced by monochromatic IR radiation: experiment and theory for CnFmHkI → CnFmHk+ I probed with hyperfine-, doppler- and uncertainty-limited time resolution of iodine-atom IR absorption

Coherent multiphoton excitation of polyatomic molecules with pulsed CO2 lasers leads to unimolecular reactions induced by monochromatic infrared radiation (URIMIR). We report a detailed study of the dynamics of dissociation of trifluoroiodomethane (CF3I), 1,1,1,2-tetrafluoro-2-iodoethane (CF3CHFI) and pentafluoroiodobenzene (C6F5I). The primary dissociation after ro-vibrational excitation in the electronic ground state results in iodine atoms I(2P3/2), which are detected by diode laser IR absorption on the (2P3/2–2P1/2) magnetic dipole transition with about 1 MHz frequency resolution and up to 1 ns time resolution, essentially bounded by the uncertainty principle. This allows us to detect the product-state distribution over nuclear hyperfine levels in I atoms, and product translational-energy distributions from Doppler lineshapes combined with quantitative, time-resolved kinetic analysis under conditions of irradiation with shape-controlled CO2 laser pulses of well defined fluence and intensity. The kinetic results for absolute rates are analysed in terms of the laser chemical rate coefficient kI(st) and compared to theoretical calculations based on the case B/C master equation including non-linear intensity effects, which are found to be important only for CF3I. The results for relative rates are analysed in terms of a simple theoretical model for the centre-of-mass product translational-energy distribution P(Et). The results are discussed in relation to the foundations of IR laser chemistry.

Intramolecular Vibrational Energy Redistribution Measured by Femtosecond Pump-Probe Experiments in a Hollow Waveguide

We report femtosecond pump-probe experiments to investigate the intramolecular vibrational energy redistribution in the gas phase for CF3CHFI, CHBrFI, CHBrClF and benzene (C6H6). To increase the measured probe signal the experiments have been performed in a hollow waveguide.