Femtosecond ionization

Hankin et al. [133] demonstrated femtosecond ionization following 266 nm desorption of solid samples of trinitrobenzene (TNB), TNT, and trinitrophenol (TNP). They confirmed the advantages of ultrafast ionization, namely, the formation of characteristic precursor and structure-specific fragment ions. The optimum intensities for efficient LD without ionization were determined for the compounds studied. Differences between femtosecond ionization of vapor samples of explosives [131,132] and laser desorbed molecules were also discussed. 

Fig. 2.12. Comparison of the mass spectra obtained by femtosecond ionization and electron impact ionization of a 2,3-dime thy 1-1,3-butadiene, and b anthracene at 1.4-pm excitation of 130-fs pulses (The El spectra are taken from NIST Standard Reference Database [29].)...

Figure 10.3 Result of time-resolved pump-probe spectroscopy in combination with TOF-MS. The graphs show femtosecond ionization signals for four DNA and RNA bases, namely cytosine, guanine, thymine, and uracil. The transient of dimethylether (DME) was displayed along with the transient of guanine for comparison [8]. Reprinted with permission from Canuel, C., Mans, M., Piuzzi, F, Tardivel, B., Dimicoli, ., Elhanine, M. (2005) Excited States Dynamics of DNA and RNA Bases Characterization of a Stepwise Deactivation Pathway in the Cas Phase. y. Chem. Phys. 122 074316. Copyright (2005) AlP Publishing EEC...

The conformer selectivity in IR-UV double resonance methods relies on the excitation bands of different conformers being resolvable in the UV spectrum. For tmresolved UV spectra, indicating either short excited state lifetimes (<1 ns) or strong conformational heterogeneity, the IR-UV ion dip scheme can be modified by introducing either a femtosecond ionization laser (IR femtosecond multiphoton ionization) [80,81] or an additional IR step (IR-IR-UV spectroscopy) [21,82-84]. 

Leon I, Montero R, Castano F, Longarte A, Femmdez JA (2012) Mass-resolved infrared spectroscopy of complexes without chromophore by nonresonant femtosecond ionization detection. J Phys Chem A 116 6798... 

Besides typical ESIPT compounds, other tautomeric systems have also been investigated by ultrafast spectroscopy. However, the number of studies is less. Cytosine was investigated by femtosecond ionization techniques to learn about the relevance of tautomerism for the photochemistry and photostabiHty of DNA [57, 58]. It was found that the isolated cytosine molecule exhibits a rather fast IC. It... 

Meier C and Engel V 1995 Pump-probe ionization spectroscopy of a diatomic molecule sodium molecule as a prototype example Femtosecond Chemistry Proc. Berlin Conf Femtosecond Chemistry (Berlin, March 1993) (Weinheim Verlag Chemie)... 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

Consider the dynamics of ionization of clusters through the C state of an ammonia molecule, where it should be noted that it is also possible that the excitation leads to some population of the B states due to the broad spectral bandwidth of the femtosecond laser pulses. The measurements indicate lifetimes... 

Figure 11. C2H4 ion yield as a function of time in femtoseconds for a pump-photoionization probe experiment. Heavy line Predicted ion yield using the AIMS data and assuming an ionization threshold of 3.5eV. Dashed line Exponential fit to the AIMS ion yield predicting an excited state lifetime of 35 fs. Gray shaded area Reported ion yield [152] obtained using an exponential fit to the experimental data predicting an excited state lifetime of 30 15 fs. (Figure adapted from Ref. 214.)...

Recently, a systematic experimental study has clearly proved the effect on a gaseous medium of both ASE and picosecond pedestal prior to the arrival of the ultrashort intense laser pulse [26]. The study has been based on sequences of electron density maps obtained from optical interferograms with femtosecond resolution and has been supported by numerical simulation of the ionization of the medium. 

The study confirmed that the action of the precursors of the main pulse has to be carefully considered in the ultrashort intense laser pulse interactions. Nevertheless, a regime of rather stable propagation [34] of a laser pulse of tens of femtoseconds was found in a broad window of the laser intensity/medium density diagram, in which (1) the ionization occurs in one or a few optical cycles, avoiding effects of self-phase modulation and defocusing for most of the... 

An alternative way to preform a channeled plasma consists in exploiting the nanosecond precursor that usually precedes a short femtosecond pulse in the output of a multi-terawatt laser system. In fact, the amplified spontaneous emission (ASE) pedestal has typically an intensity 106-1010 times lower than the main pulse, which, however, can be sufficient to ionize a gas-jet or a solid target. This drawback can be turned into a benefit assuming that this long precursor can prepare the plasma channel for the short pulse propagation. 

Y. Nosenko, M. Kunitski, R. P. Thummel, A. Kyrychenko, J. Herbich, J. Waluk, C. Riehn, and B. Brutschy, Detection and structural characterization of clusters with ultrashort lived electro nically excited states IR absorption detected by femtosecond multiphoton ionization. J. Am. Chem. Soc. 128, 10000 (2006). 

Seel, M., and Domcke, W. (1991), Femtosecond Time-resolved Ionization Spectroscopy of Ultrafast Internal-Conversion Dynamics in Polyatomic Molecules Theory and Computational Studies, J. Chem. Phys. 95,7806. 

Most of these processes are very fast. Ionization happens on the low femtosecond timescale, direct bond cleavages require between some picoseconds to several tens of nanoseconds, and rearrangement fragmentations usually proceed in much less than a microsecond (Fig. 5.3 and Chap. 2.7). Finally, some fragment ions may even be formed after the excited species has left the ion source giving rise to metastable ion dissociation (Chap. 2.7). The ion residence time within an electron ionization ion source is about 1 ps. [9]... 

Formaldonitrone, CH2=N(H)—O (3), the elusive simplest organic nitrone, has been prepared transiently in the gas phase by femtosecond collisional neutralization of its cation radical, CH2—N(H)—0+". The latter was generated by dissociative ionization of 1,2-oxazolidine. Nitrone 3 showed negligible dissociation upon collisional neutralization and was distinguished from its tautomers formaldoxime 2 and nitrosomethane 1 that gave different NR mass spectra. The enthalpy of formation was calculated from enthalpies of atomization and two isodesmic reactions as Af//29s(3) = 58 1 kJmol . The calculated, large activation barriers for isomerization of 3 (179 and 212 kJmoH for 3 anti-2 and 3 1, respectivelyindicate that once 3 is formed and diluted in the gas phase it should not isomerize unimolecularly to either 1 or (syn/anti) 2. 

Recently, laser multiphoton ionization of solutes has been used. Defining the threshold of ionization, iJth, can be a problem in some of these methods. A recent multiphoton technique, utilizing femtosecond laser pulses, appears to give quite accurate thresholds [56]. In this work, a conductivity spectrum is measured at visible wavelengths and a sharp drop in current occurs as the mechanism changes from -photon excitation to (n+ l)-photon excitation, where n is typically 3 to 4. The threshold is defined by fitting the current to an analytic function that defines the midpoint of this transition. Eth is then n times the energy at which the midpoint occurs. The thresholds are sensitive to Vg and could be used for determination of this quantity. 

The pump-probe-detect arrangements for the femtosecond experiments was similar to those described above. When cyclobutanone was pumped with two photons of a X = 307-nm femtosecond pulse, two consecutive C—CO bond cleavages led to the formation of the trimethylene diradical, detected as an easily ionized transient at 42 amu, with buildup and decay times of 120 20 fs. The decay presumably involves isomerizations to cyclopropane and to propylene— structures not ionized by the probe pulse and thus undetected during the experiment. 

The mass spectrum of diazabicyclo[2.2.1]hept-2-ene shows only a weak molecular ion and a very strong fragment at 68 amu. The femtosecond studies found that the 68 amu easily ionized transient profile could be modeled with a rise time of 30 10 fs and decay time of 190 10 fs, a value comparable to the decay time of trimethylene. 

Cyclohexene, upon excitation through a two-photon process providing 186 kcal/ mol, gives two species detected through ionization by a probe pulse and mass spectrometry a species at 82 amu, the parent stmcture or the drradical species formed through p-cleavage, and at 54 amu, a mass corresponding to butadiene. An ion at M-15, at 67, is also recorded. The femtosecond transients show that the 82 amu... 

The ionization of focal volume, or formation of plasma, is expected to alter the usual photochemical material modihcation pathway, as has been recently demonstrated in photopolymerization of SU-8 resist by femtosecond pulses [57]. In addition, nanometric-sized plasma regions created by the ionization, e.g., at defect sites, have spatio-temporal dynamics of their own. Recently, a model of nanosheet formation from plasma nanospheres in glass has been proposed [60]. Similar conditions are expected in polymers as well. Let us discuss here held enhancement by a metalhc nanoparticle (similar arguments are also valid for surfaces containing nanometric features). 

In Section 6.3.2, we presented experimental data from strong-field excitation and ionization of K atoms with shaped femtosecond laser pulses. Here we give a description of the apparatus and strategy used in the experiments presented in this contribution. Figure 6.12 gives an overview over the complete experimental two-color setup. For the experiments on strong-field control of K atoms (cf Sections 6.3.2.2, 6.3.2.3, and 6.5) only the one-color beamline was used. An... 

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