Pump and ionize

Fig. 2.32. Schematic of the ion trap used for the real-time NeNePo experiments. The first quadrupole prepares a beam of mass-selected negative ions (here Agn). These are stored in the central quadrupole ion trap. Here, detachment (pump) and ionization (probe) laser pulses interact with the stored molecules or clusters. The positively charged ions are subsequently accelerated to a third quadrupole, where again mass filtering can be carried out. The distance the Agn cluster runs on the trajectory between pump and probe pulses is greatly exaggerated. Over a time of a few picoseconds the cluster stays nearly in the same position, since its velocity is rather small...

The sample should be liquid or in solution. It is pumped and nebulized in an argon atmosphere, then sent through a plasma torch that is, in an environment where the material is strongly ionized resulting from the electromagnetic radiation produced by an induction coil. Refer to the schematic diagram in Figure 2.8. 

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)... 

For IBSCA analysis, standard HV or, better, UHV-equipment with turbomolecular pump and a residual gas pressure of less than 10 Pa is necessary. As is apparent from Fig. 4.46, the optical detection system, which consists of transfer optics, a spectrometer, and a lateral-sensitive detector, is often combined with a quadrupole mass spectrometer for analysis of secondary sputtered particles (ions or post-ionized neutrals). 

The delay time between the pump and the probe laser pulses is usually very short in these experiments. The delay time is less than 5 ns when the pump and the probe laser pulses are the same, and the delay time is as long as several hundred nanoseconds when the pump and the probe laser pulses are from two different sources. The short delay time ensures that the fragments flying with different velocities are equally sampled before they leave the detection region. Since the delay time is much shorter than the lifetime of the excited molecules (.A ), most of these molecules do not dissociate into fragments when the probe laser pulse arrives. As a result, the probe laser can easily cause dissociative ionization of the vibrationally excited molecules due to their large internal energy. 

The possible profiles of intensity versus the delay between the pump and probe photons for the several potential processes operative in the mechanism of ionization are shown in Figures 6a, b, and c. Several different laser schemes have been employed to investigate the reaction dynamics pertaining to the formation of protonated ammonia clusters through the A and C states (the latter possibly including the B state as well). The ionization schemes employed in the present study are shown in Figure 7. 

The particle beam system is a simple transport device, very similar to a two-stage jet separator. The solvent vapour is pumped away, while the analyte particles are concentrated in a beam and allowed to enter the mass spectrometric source. Here they are vapourized and ionized by electron impact. 

The pump and probe pulses employed may be subjected to a variety of nonlinear optical mixing processes they may be prepared and characterized by intensity, duration, spectral band width, and polarization. They may arrive in the reaction chamber at a desired time difference, or none. The probe pulse may lead to ionizations followed by detections of ions by mass spectrometry, but many alternatives for probing and detection have been used, such as laser-induced fluorescence, photoelectron spectroscopic detection, absorption spectroscopy, and the like. 

Following another experimental approach, GWgoire et al [9] have tried to understand the influence of an increasing number of solvent molecules on the femtosecond dynamics of diatomic molecules, including the dimers Nal and Csl. Due to its relative simplicity, the isolated Nal molecule has been studied extensively with pump-probe techniques both experimentally [10], and theoretically [11,12], In this report, we investigate theoretically the femtosecond pump-probe ionization of Nal and Csl when aggregated with a molecule of acetonitrile CH3CN. 

For a three-photon and a two-photon process we have shown that vibrational wavepacket propagation excited by an ultrashort laser pulse can be used to drive a molecule to a nuclear configuration where the desired product formation by a second probe pulse is favored (Tannor-Kosloff-Rice scheme). In both cases the relative fragmentation and ionization yield of Na2 was controlled as a function of pump-probe delay. By varying the delay between pump and probe pulses very slowly and therefore controlling the phase relation between the two pulses, additional interference effects could be detected. 

Figure 1. Experimental set-up for performing transient two-photon ionization spectroscopy on metal clusters. The particles were produced in a seeded beam expansion, their flux detected with a Langmuir-Taylor detector (LTD). The pump and probe laser pulses excited and ionized the beam particles. The photoions were size selectively recorded in a quadrupole mass spectrometer (QMS) and detected with a secondary electron multiplier (SEM). The signals were then recorded as a function of delay between pump and probe pulse.

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