Experimental Basics

Femtosecond real-time spectroscopy requires sophisticated experimental and theoretical techniques. Some experimental basics are given in this chapter. A few remarks on the theoretical basics relevant to the investigations presented in this book are included in the second part of this chapter. It should be pointed out that this treatment is intended to give a rapid overview but is not at all complete. Those readers who are looking for a more detailed description of these fascinating experimental and theoretical techniques should therefore consult the references given in Sects. 2.1 and 2.2. 

The success of surface research is based on the so-called surface science approach, that is, the investigation of well-defined and clean single crystal surfaces under ultrahigh vacuum (UHV) conditions using a seemingly unlimited arsenal of highly surface-sensitive techniques that were developed continuously over the past four decades. UHV environment is not only necessary to maintain the state of a surface - once cleaned - for the duration of the experiment but also a condition for the application of many of the surface analytical methods, which are often based on electron, ion, or atom beams. Colhsions between these beam particles and residual gas atoms or molecules must be minimized in order not to disturb the measurements. 

UHV refers to pressures lower than 10 Pa. Some more recent research directions demand an even cleaner environment, namely extreme high vacuum (XHV). This implies an even higher technological level, in particular, a careful choice of vacuum-compatible materials and advanced solutions of generating and measuring these low vacua. A classification of degrees of vacuum is presented in Table 3.1.1. For comparison, much more ideal vacuum conditions are found in deep space where typical interstellar pressures are of the order of 10 Pa. 

The intention of this chapter is to introduce first the physical bacl ound of vacuum by recalling some aspects of the theory of gases. Furthermore, the creation and maintenance of pressure gradients, that is, the process of pumping, is considered. Next, the necessary UHV technology is introduced, and the standard ways of achieving and maintaining certain vacuum levels are described. Also, 

1) Other commonly used units are millibar and Ton. 1 Torr = 133,3 Pa, 1 mbar = 

Surface and Intetfiice Science Concepts and Methods, First Edition. Edited by Klaus Wandelt. 

---

Earlier comparisons of experimental basicities toward H+ and Li+ have led to the conclusion that there is no precise general correlation between LCBs and GBs, especially when diverse families of compounds with different functional groups are included (Figure 6). 

Fig. 1. Relationship between Pauling electronegativity and the experimental basicity moderating...

TABLE 16. Experimental basicities in the gas-phase [PA of equation 9 (kcal mol )] determined by ICR, and pKa in solution in water at 25°C, of amines and enamines shown in Scheme 8... 

The interfacial (excess) heat capacity is another Interfacial characteristic that we decided to disregard. The reason for doing so is not in its intrinsic interest. On the contrary, as with bulk heat capacities, they reflect the structure, or ordering (see e.g. FIGS I, sec. 5.3c). However, it is very difficult to establish these values experimentally. Basically the second derivative of the interfacial tension with respect to the temperature at a constant pressure is needed (see sec. 1.2.7), and to obtain this extremely preeise measurements are needed. The spread in the quadratic coefficient B in [1.12.1) indicates the uncertainty, even for a well-studied liquid like water. Yang and Li showed, by a thermodyncimic ancdysis, that for LL interfaces this heat capacity is related to the two bulk heat capacities, the inter-... 

The three experimental basic coefficients A, B and C are related to diverse physico-chemical parameters of the column and to the experimental conditions. If H is expressed in cm, A will also be in cm, B in cm /s and C in s (where velocity is measured in cm/s). 

Absolute Proton Affinities 2.1. Experimental Basicity Scales... 

Capron, A. Human Experimentation Basic Issues. The Encyclopedia of Bioethics,YdL II. The Free Press, Glencoe, II. 1978. 

The principles of two experimental setups are described in this section. In Sect. 2.1.1 the setup for the real-time MPI experiments is introduced. With this setup, the investigations discussed in Chaps. 3 and 4 were carried out. The experimental basics are concluded with a brief description (Sect. 2.1.2) of the setup used for the real-time studies of charge reversal processes discussed later in Chap. 5. 

The question now is In spite of significant solvent effects, are the solution Lewis basicity scales closely related to the intrinsic gas-phase Lewis basicity scales This is an important question for computational chemists who need to identify the computational methods that yield reliable basicities. A relative comparison of gas-phase computed basicities with solution experimental basicities would avoid the difficult and approximate modelling of the solvent effect [105]. However, this comparison requires that experimental gas-phase and solution basicities (affinities) be strongly correlated. This correlation appears to exist for BF3 affinities and hydrogen-bond basicities. Equation 1.96... 

In addition, measured experimental basicities and affinities are essential to computational chemists for the validation, calibration and establishment of reliable computational methods for quantifying and explaining intermolecular forces and the chemical bond. In fact, the formation of Lewis acid/base adducts covers a wide variety of bond-forming processes from the weak hydrogen bond to the strong dative bond or ion/molecule bond. 

A gas-phase lithium cation basicity scale has been calculated using G2 and G2(MP2) on 37 compounds and B3LYP/6-311-I-G calculations of 63 compounds including Lewis bases and saturated and unsaturated organic molecules. Good agreement with experimental basicities is found for all three... 

---