Collisional resonances atoms

One of the more interesting aspects of the collision process is the calculated dependence on the orientation of v relative to E. In particular, do the lineshapes for collisions with v E and v 1E differ as dramatically as shown in Fig. 14.6 To answer this question unambiguously required two improvements upon the initial measurements. First, the Na atoms must be in a well defined beam, otherwise v is not well defined. This requirement is easily met by enclosing the interaction region with a liquid N2 cooled box, which ensures that the only Na atoms in the interaction region are those in the beam. Second, the field homogeneity must be adequate, 1 part in 104, to resolve the intrinsic lineshape of the collisional resonances. A pair of Cu field plates 1.592(2) cm apart with 1 mm diameter holes in the top plate to allow the ions to be extracted is adequate to meet this requirement. 

One of the potentially most interesting aspects of the resonant collisions is that, in theory, the collision time increases and the linewidth narrows as the collision velocity is decreased. According to Eqs. (14.6) and (14.8) the collision time is proportional to l/v3/2. Collisions between thermal atoms with temperatures of 500 K lead to linewidths of the collisional resonances that are a few hundred MHz at n = 20. In principle, substantially smaller linewidths can be observed if the collision velocity is reduced. 

The first and most obvious question is whether or not a narrower velocity distribution leads to narrower collisional resonances. In Fig. 14.14 we show the Na 26s + Na 26s — Na 26p + Na 25p resonances obtained under three different experimental conditions.20 In Fig. 14.14(a) the atoms are in a thermal 670 K beam. In Figs. 14.14(b) and (c) the beam is velocity selected using the approach shown in Fig. 14.13 to collision velocities of 7.5 X 103 and 3.8 X 103 cm/s, respectively. The dramatic reduction in the linewidths of the collisional resonances is evident. The calculated linewidths are 400, 28, and 10 MHz, and the widths of the collisional resonances shown in Figs. 14.14(a)-(c) are 350,40, and 23 MHz respectively. The widths decrease approximately as l/v3/2 until Fig. 14.14(c), at which point the inhomogeneities of the electric field mask the intrinsic linewidth of the collisional resonance. 

To describe the shifts and intensities of the m-photon assisted collisional resonances with the microwave field Pillet et al. developed a picture based on dressed molecular states,3 and we follow that development here. As in the previous chapter, we break the Hamiltonian into an unperturbed Hamiltonian H(h and a perturbation V. The difference from our previous treatment of resonant collisions is that now H0 describes the isolated, noninteracting, atoms in both static and microwave fields. Each of the two atoms is described by a dressed atomic state, and we construct the dressed molecular state as a direct product of the two atomic states. The dipole-dipole interaction Vis still given by Eq. (14.12), and using it we can calculate the transition probabilities and cross sections for the radiatively assisted collisions. 

When the phase of a low frequency rf field is controlled the observed collisional resonances change dramatically. With the field of Eq. (15.32), when the atoms are allowed to collide during the interval -0.5 fis < t < 0.5 pts with 0 = 0 or tt, we observe the collisional resonances shown in Figs. 15.12(b) and (c) respectively, in agreement with Eq. (15.33). With no rf field the resonance occurs at Es = 6.44... 

An interesting aspect of the collisional resonances shown in Fig. 2 is that they are quite easy to observe. In fact, most of the Rydberg atoms undergo collisions, and we can estimate the cross section rather easily using... 

Eq. (8) is given in atomic units, and if we re-express it in laboratory units, for n = 20, we find a = 3.10-8 cm2 and r = 10-9 s. This value of the cross section is in accord with our earlier rough estimate, and the inverse of the collision time is consistent with the linewidths of the collisional resonances shown in Fig. 2. The n scaling of both the cross section and the resonance width has been verified, and in Fig. 3 we show the observed dependence of the width A of the (0, 0) resonance on n [Gallagher 1982],... 

One of the more interesting aspects of the collisional resonances is how sharp they are. Most atomic collision processes have durations of 10-12 s, and these collsions last for times in excess of a nanosecond. Furthermore, the collisional resonances should become even narrower as the collision velocity is reduced. To explore this issue we used the K system... 

A classical expression for the cross section for collisional de-excitation of He(2 P) is also derived from the formula by Eq. (16). However, the autoionization widths r(R) for Penning ionization by resonant atoms are not identical to the empirical form of Eq. (18) for electron exchange. Instead, a direct transition due to a dipole-dipole interaction is proposed to govern this Penning ionization [126,139,140,143], that is. 

Collisional or pressure broadening and resonance broadening. These are caused by collisions between unlike and like atoms respectively in the sample vapour. Only the former is significant in flames. 

One easily understood mechanism for changes in lifetime is collisional quenching (Figure 10.3). A variety of substances act as quenchers, including oxygen, nitrous oxide, heavy atoms, Cl , and amines, to name a few. By consideration of the lifetime in the absence (to) and presence (r) of collisional quenchers (no resonance energy... 

Collisional redistribution of radiation. A system A + B of two atoms /molecules may be excited by absorption of an off-resonant photon, in the far wing of the (collisionally) broadened resonance line of species A. One may then study the radiation that has been redistributed into the resonance line - a process that may be considered the inverse of pressure-broadened emission. Interesting polarization studies provide additional insights into the intermolecular interactions [118, 388]. 

The increased cross sections for these three states are attributed to resonant electronic to vibrational energy transfer. Table 11.1 identifies the three atomic transitions and the resonant molecular transitions in CH4 and CD4. For example the rapid depopulation of the Na 7s state by CD4 is attributed to the Na 7s — 5d transition. To verify this assignment the cross section for the 7s — 5d transfer was measured for both CH4and CD4 by observing the 5d-3p fluorescence as well as the 7s-3p fluorescence. The 7s — 5d cross sections are 215 A2 for CD4 and 15 A2 for CH4. As shown by Fig. 11.16, the 7s CD4 cross sections is —240 A2 above the smooth dotted curve in good agreement with the 7s — 5d cross section. Similar confirmations were carried out for the other two resonant collisional transfers. 

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