P. P. Radi

Determination of rotational constants in a molecule by femtosecond four-wave mixing

Femtosecond time-resolved DFWM experiments were performed on CHCl3 and SO2. The recurrences observed originate from the preparation of rotational coherences within the sample. The determination of rotational constants for a symmetric and an asymmetric top are shown. The simulation of the transients for the symmetric top allows the extraction of the main moment of inertia B and the centrifugal terms perpendicular to the molecule axis. For the asymmetric top, all three rotational constants can be derived. In addition to the transients at 1/2(B + C), two additional types of asymmetry transients at 1/4C and 1/4A arise from this experiment. The first origins from the levels at low K and the second from the levels at high K in the high-J limit. Copyright

Collision induced rotational energy transfer probed by time-resolved coherent anti- Stokes Raman scattering

We show that the technique of femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy provides a powerful tool for the investigation of collision-induced linewidths and the validation of rotational energy transfer (RET) models. The fs-CARS method is applied to the N2–N2 collision system, and a comparison between the commonly used exponential gap (ECS-E), power gap (ECS-P), frequency corrected (EFCS), and the recently proposed angular momentum and energy corrected (AECS) variants of the ECS model is presented. As result we show that the AECS scaling law requires only two free parameters, and is appropriate for the determination of RET rates from the measured fs-CARS signals. The AECS model is also applied to the more complex C2H2–C2H2 collision system. As vibrational energy transfer and dephasing is not negligible in this case, the model has to be modified by introducing a vibrational relaxation factor. With this modification the fs-CARS signals from acetylene can be described s...

Stimulated emission pumping of OH and NH in flames by using two-color resonant four-wave mixing

Abstract In this work we examine the analytical potential of two-color resonant four-wave mixing for the determination and characterization of trace elements in a combustion environment. Experimental results for NH and OH in flames at atmospheric pressure are presented. The selectivity of the technique is used to simplify the Q-branch region of the (0-0) A3Π−X3Σ− vibronic transition of NH. Furthermore, substantial signal-to-noise ratios in the (0-0) A2Σ+−X2Π system of OH are achieved. The high sensitivity is applied to perform stimulated emission pumping involving the weak (0–1) vibrational band. In addition, we demonstrate that the technique is sensitiive to state changing collisions.

Nearly degenerate four-wave mixing using phase-conjugate pump beams

A novel configuration has been developed for performing degenerate four-wave mixing (DFWM) experiments using optical phase conjugation from stimulated Brillouin scattering in a cell. The typical geometry for DFWM requires that two pump beams counterpropagate through the measurement volume, which is accomplished either by splitting off a portion of the beam and directing it in from the opposite direction or reflecting the first pump beam back onto itself. In both approaches it can be difficult to maintain alignment between the counterpropagating pump beams, particularly in combustion systems that present thermal and density gradients. DFWM has been demonstrated for OH in a flame by using counterpropagating pump beams that are phase conjugate. The optical phase conjugation is achieved by focusing the first pump beam into a cell containing hexane, which produces a counterpropagating beam through stimulated Brillouin scattering. This approach greatly simplifies DFWM experiments by relaxing alignment constraints and sensitivities to index-of-refraction gradients in the flow.

Charged particle velocity map image reconstruction with one-dimensional projections of spherical functions

Velocity map imaging (VMI) is used in mass spectrometry and in angle resolved photo-electron spectroscopy to determine the lateral momentum distributions of charged particles accelerated towards a detector. VM-images are composed of projected Newton spheres with a common centre. The 2D images are usually evaluated by a decomposition into base vectors each representing the 2D projection of a set of particles starting from a centre with a specific velocity distribution. We propose to evaluate 1D projections of VM-images in terms of 1D projections of spherical functions, instead. The proposed evaluation algorithm shows that all distribution information can be retrieved from an adequately chosen set of 1D projections, alleviating the numerical effort for the interpretation of VM-images considerably. The obtained results produce directly the coefficients of the involved spherical functions, making the reconstruction of sliced Newton spheres obsolete.

Femtosecond photoionization of (H2O)n and (D2O)n clusters

Cluster ion distributions of water in a molecular beam are investigated by femtosecond ionization at 780 nm and reflectron time-of-flight mass spectrometry. The electric field strength generated by the ultrashort laser pulses is sufficient to efficiently ionize most of the molecules that are present in the molecular beam. In this work ion signals of large water clusters containing up to 60 monomers are reported. Upon ionization rapid proton transfer is observed, leading to the formation of protonated water cluster ions. Unprotonated clusters (H2O)n+(n>2) are not observed in the mass spectra. The configurational energy imparted to the protonated clusters induces unimolecular dissociation on the μs time scale. These metastable reactions are characterized by modeling the ion trajectories in the mass spectrometer. The numerical procedure in conjunction with the integrated parent and daughter intensities results in unimolecular dissociation rates as a function of cluster size. Additional information about prot...

Resonant two-photon ionization of LiNa. Observation and preliminary characterization of five new singlet states

Abstract Supersonic molecular beams containing rotationally and vibrationally cold LiNa were probed by one- and multi-photon ionization. Results include determination of a vertical ionization potential (5.05 ± 0.04 eV) as well as first observation of five new singlet states. Preliminary spectroscopic constants ( T e, w e and w e x e ) and term symbols are reported for these five states (A 1 Σ + , C 1 Σ + , D 1 Π, E 1 Σ + and F 1 E + ).

Absolute concentration measurements using DFWM and modeling of OH and S2 in a fuel-rich H2/air/SO2 flame

Concentration profiles of OH and S{sub 2} in a sulfur-containing premixed H{sub 2}/air flame were obtained and compared to numerical calculations. The measurements utilized degenerate four-wave mixing and absorption spectroscopy in parallel (MULTIPLEX spectroscopy) and were numerically simulated for a one-dimensional flat flame. An established reaction mechanism for hydrogen oxidation was extended by reactions for the sulfur chemistry drawn from the literature. The experimental results for the OH and S{sub 2} concentration profiles are in good qualitative agreement with the simulations. However, the model calculations underestimate the S{sub 2} concentrations by two orders of magnitude, indicating that important intermediates and reactions could have been omitted.

Electronic spectra of radicals in a supersonic slit-jet discharge by degenerate and two-color four-wave mixing

Four-wave mixing techniques have been used for the measurement of electronic transitions of cold transient species generated in a supersonic slit-jet discharge expansion. The origin band of the d(3)Pi(g)-a(3)Pi(u) system of C(2) and A(2)Pi(3/2)-X[combining tilde](2)Pi(3/2) electronic transition of HC(4)S were recorded. A signal-to-noise ratio of 10(4) in the spectra was achieved, resulting in detection limits of 10(10) cm(-3) for these two molecules. Application of selective two-color resonant four-wave mixing is used for the spectral assignment utilizing the double-resonance nature of the method. The combination of these techniques with a slit source proves to be a sensitive approach for the detection of transient molecules in a molecular beam discharge.

Real-time observation of ultrafast internal conversion in ethylbenzene by femtosecond time-resolved photoelectron imaging

The ultrafast dynamics of the second singlet electronically excited state (S2) in ethylbenzene has been studied by femtosecond time-resolved photoelectron imaging. The time evolution of the photoelectron signal can be well described by a biexponential decay: a rapid relaxation pathway with a time constant of 60 ( ± 9) fs and a longer-lived channel on a timescale of 2.58 ( ± 0.22) ps. The rapid relaxation is ascribed to the ultrafast internal conversion from the S2 state to the vibrationally hot S1 state. This internal conversion process has been observed in real time. The slow photoelectron signal reflects the depopulation of secondarily populated high vibronic S1 state.