TOTAL VENTAS E INGRESOS Y POR FUNCIONES ASISTENCIALES

Monitoring the evolution of fragment ion formation in time is an extension to photo- induced dissociation, which is realized by a pump-probe scheme. The advent of ultrafast lasers paved the way for time-resolved spectroscopy on the timescale of chemical reactions (femtochemistry) pioneered by A. H. Zewail (Nobel prize in chemistry in 1999).[41] _{In these early experiments a pump-probe scheme was employed to study} photodynamics of small, neutral molecules in the gas phase by multiple-photon- ionization on the pico- and femtosecond timescale.[42-43] _{The molecules were excited to a} higher electronic state by a pump laser and subsequently ionized from the excited state by probe photon absorption. Although the photoionization scheme is common in pump- probe spectroscopy, it is generally not applicable to cationic species, as the higher ionization potentials of cations are difficult to overcome. Hence a different detection scheme is employed, monitoring the fragmentation efficiency as a function of pump- probe delay. This particular scheme was first realized by Jouvet et al. in their studies on protonated tryptamine[44] _{and tryptophan}[45] _{with time-of-flight (TOF) detection of} fragment ions, and later expanded to commercially available ion trap mass spectrometers by Weinkauf et al.[27-28]

The transient pump-probe PF (tPF) method is based on interrogating the population of a resonantly excited (pump step) molecular ensemble, e.g. excited into the first electronic excited state S1, by probe photon absorption, inducing a subsequent transition to higher lying electronic states (Sn←S1), which is accompanied by an increase in fragmentation yield. As discussed for the steady-state case, the molecules do not necessarily fragment directly from the respective Sn state, but later-on after IC from a highly vibrationally excited ground state. In ion trap setups, fragmentation from the ground state takes place in competition to collisional quenching with the helium buffer gas on a µs-ms time scale, depending on the partial pressure.[46] _{The increase in fragmentation rate and yield stems} from the increase in internal energy of molecules after pump+probe excitation with respect to pump-only excitation. Noteworthy, the probe process is not necessarily controlled to be resonant and assumed to be of multi-photonic character, thus its underlying transitions to higher-lying electronic states are in general unknown. However, when the molecular system undergoes electronic relaxation processes (e.g. IC or intersystem crossing, ISC) the cross section of the probe pulse is altered between the

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different states, usually resulting in a decreased absorption cross section upon formation of lower lying electronic states. This is attributed to three effects: 1) a dilution of population, due to population of several vibronic states, 2) a loss of propensity, i.e. Franck-Condon overlap, for these vibronic states towards higher-lying electronic states, and 3) the multiple-photon character of the probe absorption process, which makes transitions from lower electronic states less likely. Figure 19 depicts a simplified tPF scheme for an arbitrary system, in which IC is the only relaxation channel after photoexcitation.

Figure 19 tPF detection scheme. Arrow colors denote different

radiative/non-radiative processes, whereas boldness represents the process rates (bolder arrows indicate higher rates or higher probabilities). Dots represent ion population in the respective electronic/vibrational state. For the sake of clarity, vibrational states of the excited states are not illustrated. Ion population is depicted for three cases: (a) negative pump-probe delay, i.e. probe arrives prior to resonant pump-excitation, (b) coherent pump-probe excitation at zero time delay and (c) positive pump-probe delay, i.e. probe arrives after pump-excitation.

33 In case a), the ion population is excited resonantly to the S1 electronic state and decays via IC to the hot ground state S0 by coupling to vibronic states from which the excited ions fragment, forming two different fragment products (F1 and F2). The relative fragmentation yields depend on the internal energy, with F2 exhibiting a higher barrier for fragmentation. Fragmentation proceeds on a longer timescale (µs-ms) than the electronic decay (ps) and is in competition with thermalization of the hot ions by collisional quenching. No excitation to higher electronic states (Sn) by probe photon absorption takes place. In case b) a majority of the pump-excited ion population is elevated to a higher lying electronic state Sn, by (multiple-)probe-photon absorption. IC takes place from either the S1 or the Sn state, resulting in two populations in S0 with distinctly different internal energies. The “hotter” population is less susceptible to collisional deactivation, thus higher total fragmentation yields are observed with respect to pump-only excitation (at the same pump energy). Case c) depicts a scenario, in which the probe pulses arrive sometime after excitation with pump-photons, so that the system had time to partially relax; in this case to the S0 state. Only the ions in the S1 state have a relatively high cross section for multiple-photon absorption of the probe photons, whereas ions in the hot S0 state are less likely to absorb probe photons (ion population is “diluted” over several vibrational states, exhibiting smaller Franck-Condon overlap). Compared to b) the ion population in the “hotter” S0 state is smaller. Hence the total fragmentation yield is also lower. With increasing pump-probe delay, the effective ion population in the Sn state diminishes, further reducing the fragmentation yield. The delay dependent fragment ion intensities thus map the lifetime of the S1 state. The fragment signal evolution sketched for a simple, arbitrary case in Figure 19 can be applied to any system, which may exhibit various relaxation channels of the excited state population.

Although the detection scheme is akin to the one employed by Jouvet et al. and Weinkauf

et al., the experimental setup utilized in this thesis has a major advantage over the

referenced setups, as the OPA systems allow for tuning of the pump and probe pulses in a broad spectral range individually, not relying on the fundamental (~800 nm) of a Ti3+_{:sapphire laser and its higher harmonics for generation of suitable laser pulses. Thus} a wider assortment of molecules can be investigated, such as organic dyes with extensive π-systems or charge transfer processes in transition metal complexes, which require pump wavelengths in the visible region.

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