V. Loriot

A femtosecond velocity map imaging study on B-band predissociation in CH3I. I. The band origin

A femtosecond pump-probe experiment, coupled with velocity map ion imaging, is reported on the second absorption band (B-band) of CH(3)I. The measurements provide a detailed picture of real-time B-band predissociation in the band origin at 201.2 nm. Several new data are reported. (i) A value of 1.5+/-0.1 ps has been obtained for the lifetime of the excited state, consistent within errors with the only other direct measurement of this quantity [A. P. Baronavski and J. C. Owrutsky, J. Chem. Phys. 108, 3445 (1998)]. (ii) It has been possible to measure the angular character of the transition directly through the observation of fragments appearing early with respect to both predissociation lifetime and molecular rotation. (iii) Vibrational activity in CH(3) has been found, both in the umbrella (nu(2)) and the symmetric stretch (nu(1)) modes, with estimates of relative populations. All these findings constitute a challenge and a test for much-wanted high level ab initio and dynamics calculations in this energy region.

A femtosecond velocity map imaging study on B-band predissociation in CH3I. II. The 201 and 301 vibronic levels

Femtosecond time-resolved velocity map imaging experiments are reported on several vibronic levels of the second absorption band (B-band) of CH(3)I, including vibrational excitation in the ν(2) and ν(3) modes of the bound (3)R(1)(E) Rydberg state. Specific predissociation lifetimes have been determined for the 2(0)(1) and 3(0)(1) vibronic levels from measurements of time-resolved I*((2)P(1/2)) and CH(3) fragment images, parent decay, and photoelectron images obtained through both resonant and non-resonant multiphoton ionization. The results are compared with our previously reported predissociation lifetime measurements for the band origin 0(0) (0) [Gitzinger et al., J. Chem. Phys. 132, 234313 (2010)]. The result, previously reported in the literature, where vibrational excitation to the C-I stretching mode (ν(3)) of the CH(3)I (3)R(1)(E) Rydberg state yields a predissociation lifetime about four times slower than that corresponding to the vibrationless state, whereas predissociation is twice faster if the vibrational excitation is to the umbrella mode (ν(2)), is confirmed in the present experiments. In addition to the specific vibrational state lifetimes, which were found to be 0.85 ± 0.04 ps and 4.34 ± 0.13 ps for the 2(0)(1) and 3(0)(1) vibronic levels, respectively, the time evolution of the fragment anisotropy and the vibrational activity of the CH(3) fragment are presented. Additional striking results found in the present work are the evidence of ground state I((2)P(3/2)) fragment production when excitation is produced specifically to the 3(0)(1) vibronic level, which is attributed to predissociation via the A-band (1)Q(1) potential energy surface, and the indication of a fast adiabatic photodissociation process through the repulsive A-band (3)A(1)(4E) state, after direct absorption to this state, competing with absorption to the 3(0)(1) vibronic level of the (3)R(1)(E) Rydberg state of the B-band.

Structural dynamics effects on the ultrafast chemical bond cleavage of a photodissociation reaction

The correlation between chemical structure and dynamics has been explored in a series of molecules with increasing structural complexity in order to investigate its influence on bond cleavage reaction times in a photodissociation event. Femtosecond time-resolved velocity map imaging spectroscopy reveals specificity of the ultrafast carbon-iodine (C-I) bond breakage for a series of linear (unbranched) and branched alkyl iodides, due to the interplay between the pure reaction coordinate and the rest of the degrees of freedom associated with the molecular structure details. Full-dimension time-resolved dynamics calculations support the experimental evidence and provide insight into the structure-dynamics relationship to understand structural control on time-resolved reactivity.

The primary step in the ultrafast photodissociation of the methyl iodide dimer

This paper shows the results of combined experimental and theoretical work that have unravelled the mechanism of ultrafast ejection of a methyl group from a cluster, the methyl iodide dimer (CH(3)I)(2). Ab initio calculations have produced optimized geometries for the dimer and energy values and oscillator strengths for the excited states of the A band of (CH(3)I)(2). These calculations have allowed us to describe the blue shift that had been observed in the past in this band. This blue shift has been experimentally determined with higher precision than in all previously reported experiments, since it has been measured through its effect upon the kinetic energy release of the fragments using femtosecond velocity map imaging. Observations of the reaction branching ratio and of the angular nature of the fragment distribution indicate that two main changes occur in A-band absorption in the dimer with respect to the monomer: a substantial change in the relative absorption to different states of the band, and, more importantly, a more efficient non-adiabatic crossing between two of those states. Additionally, time resolved experiments have been performed on the system, obtaining snapshots of the dissociation process. The apparent retardation of more than 100 fs in the dissociation process of the dimer relative to the monomer has been assigned to a delay in the opening of the optical detection window associated with the resonant multiphoton ionization detection of the methyl fragment.

Strong field control of predissociation dynamics

Strong field control scenarios are investigated in the CH3I predissociation dynamics at the origin of the second absorption B-band, in which state-selective electronic predissociation occurs through the crossing with a valence dissociative state. Dynamic Stark control (DSC) and pump-dump strategies are shown capable of altering both the predissociation lifetime and the product branching ratio.

Experimental Demonstration of the Quasi-Direct Space-to-Time Pulse Shaping Principle

A new strategy of pulse shaping based on the quasi-direct space-to-time (QDST) conversion principle has been recently proposed theoretically [Mínguez-Vega , Opt. Express 16, 16993 (2008)]. In this letter, the working principle of this pulse shaper has been experimentally verified by using a simple second order cross-correlation setup with spatial resolution. The QDST conversion principle is evidenced through the experimental measurement of the parabolic dependence of the time delay on the focal plane versus the radii of the components of the spatial profile in the far field.

Femtosecond Photodissociation Dynamics by Velocity Map Imaging. The Methyl Iodide Case

The introduction of time-resolved measurements in the femtosecond time-scale using velocity map imaging techniques of charged particles (ions and photoelectrons) in combination with resonant multiphoton ionization of the fragments for the study of the photodissociation dynamics of small polyatomic molecules is reviewed. A typical experiment consists of the measurement of a sequence of images, whose analysis requires in most cases sophisticated multidimensional fitting methods to extract all the relevant time-resolved information contained in the images. In particular, the application of these techniques to the study of the direct photodissociation (A band) and electronic predissociation (B band) of methyl iodide along with the detection and characterization of transient species and the study of cluster dissociation, as a case example for femtosecond velocity map imaging, are presented and discussed.

Single diffractive optical element pulse shaper

Summary form only given. Pulse-shaping technology has been acknowledged as a convenient framework for synthesizing user-defined temporal waveforms. In this context, compact, dynamic, user-friendly and fast-operation devices demanded in photonics, quantum dynamics, or ultrafast telecommunications have inspired researchers to look for simple pulse-shaper designs. In this contribution, we experimentally demonstrate pulse shaping of femtosecond pulses by using a single diffractive element encoded into a phase-only spatial light modulator (SLM) [1]. In this proposal the entire size of the pulse shaper is very small, there is almost no need for optical alignment, and obviously no extra refractive focusing element is required. This type of diffractive pulse shaper can generate bursts of flattop pulses [2] with potential application, i.e., as optical packet headers in packet-switched networks, and for the photonically assisted generation of microwave or adaptive quantum control experiments. Today, similar implementations of this device have been successfully used as a versatile, and real-time tunable optical filter [3], to generate arbitrary waveforms [4] or to synthesize sequences of fractal light pulses in the femtosecond regime [5].The output of the pulse shaper is located in the close vicinity (by few micrometers) of the focal spot of a patterned kinoform diffractive lens (DL). Its temporal amplitude is approximately given by the convolution of the initial ultrashort pulse with the transmittance of certain complex mask in r2 transformed into the time domain. To measure the temporal response of the shaper an intensity cross correlation, with a /.3 -BaB2O4 (BBO) type I non-linear crystal of 70 μm thickness, between the shaped and the reference ultrashort light pulse has been carried out. In Fig.1(a) and 1(b) experimental results for a sequences of two pulses with different peak intensity and three pulses with a slowly decreasing peak intensity are shown.In order to guarantee a constant response over the whole temporal window of the pulse shaper the amplitude of the complex mask was changed radially to compensate for efficiency losses due to the finite number of pixels of the SLM. The corresponding radial phase profiles of the complex masks used in Fig. 1(a) and 1(b) are shown in Fig1(c) and 1(d), respectively. In addition, an active correction of the wavefront aberrations at the BBO crystal plane was done, see in Fig. 1(e) an example of a distorted focus, whereas in Fig. 1(f) the corresponding correction using a commercial Shark-Harmman wavefront sensor. Please, note that the SLM is not only used to encode the complex phase mask together the DL, but also to correct for wavefront distortions.