Stepwise excitation

Assume that an excited level Ej has been selectively populated by optical pumping with a laser (Fig.8.28). If the sample is irradiated with the spectral continuum of a broad-band source, the total absorption spectrum of molecules in all levels is obtained by measuring the transmitted intensity dispersed by a spectrometer (see Sect.8.1). When the intensity I of the pump laser is chopped, the specific absorption of molecules in level Ej can be selected by a lock-in detector tuned to the chopping frequency. The spectrometer may be spared if the continuum source is replaced by a tunable laser 12 The difference in the absorption 612(0 2) = a(o)2)Axl2 with and without the pump laser gives the absorption spectrum of molecules in the excited level E directly. More sensitive is the excitation spectroscopy (Sect.8.2) where the fluorescence intensity ifi (0)2) induced by L2 is monitored as a function of the frequency 0 2 the tunable laser L2 (Fig.8.28b). 

This two-step excitation may be regarded as a resonant case of the more general two-photon absorption (see Sects.8.10 and 10.6). Here the intermediate virtual level coincides with a real molecular level. Since the upper level of the two-photon absorption must have the same parity as the initial ground state, it cannot be reached by single-photon absorption. 

The fluorescence induced by the second laser allows the accurate determination of the molecular parameters for those excited states on which the fluorescence transitions from are terminating. The LIF method can therefore be extended by stepwise excitation to the investigation of many molecular states which may not even have been found before. Of particular interest are dissociating excited states with repulsive potential curves below bound states E. These continuous states often cannot be studied by direct absorption from the ground state because the Franck-Condon factors for the transitions may be quite small. As an example of such investigations we mention the two-step excitation of the iodine molecule I2 (Fig.8.29). Selected (V, J ) levels in the B rig state are populated by optical pumping with a cw dye laser. Starting from these levels a krypton laser excites further levels in a 

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Fig. 16.8 Schematic illustration of the magnetic resonance technique used to measure the Rb nf fine structure intervals. The Rb atoms in the n2F states are populated by spontaneous decay of the n D5/2 states, which are populated by stepwise excitation of the ground state atoms. The rf transitions, induced among the magnetic sublevels of the n2F states, are detected as a change in the intensity of the polarized n2F — 42D fluorescence. The lower part of the figure shows a sketch of the experimental arrangement (from ref. 36).

Figure 1 shows a portion of the data obtained for the stepwise excitation of Na. Figure 2 plots the observed signal enhancement (defined as signal with Xi + X2 divided by the signal with Xj only) versus the absorption coefficient for the stronger (3p3/2 nd) of the two components. 

Figure 1. Optogalvanic signal for stepwise excitation of sodium (3s - 3p — nd, ns) in an H,-air flame. Each transition is split into two components by the fast mixing of the fine structure states, 3p,/t — 3pi/t. The data are not normalized for the variation of laser power with wavelength. At this level of sensitivity the one-photon signal (3s —> 3p) is undetectable.

Figure 2. Comparison of the- stepwise excitation results (O) with the model calculation ( ). The enhancement (the two-photon signal divided by the one-photon signal) normalized for laser energy is plotted against the absorption coefficient for the 3p -> nd transitions. For visual clarity a curve is drawn through the points of the model calculation and a dashed line of unit slope is drawn through the data at high principal quantum number, n.

On the other hand, in thermal reduction, the vibrational stepwise excitation processes play a vital role in reaching higher vibrational levels. Therefore, it can be said that the thermal reduction of the oxide is initiated by... 

The periodic table shown in Fig. 2 indicates the elements which have been determined in analytical flames by LEI spectrometry to date. Limits of detection are given in nanograms of analyte per milliliter of distilled water aspirated into the flame. One ng/ml corresponds to to an atom density of approximately 108/cm3 in the flame 24). The table also shows whether the element was determined using a single wavelength or stepwise excitation scheme. The possible photoexcitation schemes for LEI are identified in Fig. 3. Fig. 2 reports LEI detection limits which are competitive with — and in many cases superior to — those obtained with other techniques of atomic spectrometry. 

Figure 3.1. Simplified mechanistic pattern of simultaneous TPA ((a) and (b)) in comparison with stepwise excitation (c) with two photons (< i and co2 — excitation frequencies, t = lifetime of the state considered, dashed line = energy distribution of the virtual state according to the Heisenberg relationship, solid line = energy levels of the real states, i.e., the ground state and the excited states, r> = absorption cross section from ground state to the virtual state, Ojf = absorption cross section from the virtual state to the final excited state), (a) Simultaneous excitation with one color, o> =

In this connection (as already hinted above), an interesting experimental possibility arises. The symmetry of an autoionising resonance (but not its width) depends on the mode of excitation. Thus, for example, for an atom with an s2 ground state, one may excite the same autoionising resonances either directly by single-photon spectroscopy, or by stepwise excitation via an intermediate, excited msns 1So state, which can be reached by a two-photon transition [441], In the latter instance, the symmetries of the lines can be dramatically altered, and in the present theory, this... 

Resonance ionization of atoms and molecules by stepwise excitation is a well-established method[26], generally applied in trace element analysis [27]. Due to the high sensitivity, the method is well suited for applications in measurements of IS and hfs in radioactive isotopes. The IS and hfs is then probed in the first step of RIS, with a narrowband laser. For the other steps broadband lasers may be used. The first demonstration was performed at the St. Petersburg mass-separator on Eu isotopes... 

A sufficiently large population of the upper state furthermore allows the measurement of absorption spectra for transitions from this state to still higher-lying levels (excited-state spectroscopy, stepwise excitation) (Sect. 5.4). Since all absorbing transitions start from this selectively populated level, the absorption spectrum is again much simpler than in gas discharges. 

Fig. 5.15 Different schemes for OODR (a) V-type OODR (b) stepwise excitation (c) A-type OODR...

The second OODR scheme (Fig. 5.15b) represents stepwise excitation of high-lying levels via a common intermediate level 2, which is the upper level of the pump transition but the lower level of the probe transition. This scheme allows the investigation of higher levels (e.g., Rydberg states), where the absorption of the probe can be monitored by either LIF from these levels, or by the ions produced by the absorption of a third photon. 

On the other hand, the large dipole moment of Rydberg atoms offers the possibility to use them as sensitive detectors for microwave and submillimeter-wave radiation [566]. For the detection of radiation with frequency to, a Rydberg level n) is selectively excited by stepwise excitation with lasers in an external electric dc field. The field strength is adjusted in such a way that the energy En of the Rydberg level n) is just below the critical value Eq for field ionization, but E -i- hco is just above. Every absorbed microwave photon tuo then produces an ion that can be detected with 100 % efficiency (Fig. 5.26). 

Until now detailed experiments on Rydberg atoms in crossed electric and magnetic fields have been performed on alkali atoms [568] and on Rydberg states of the H atom [573], which are excited either by direct two-photon transitions or by stepwise excitation via the 2 p state. Since the ionization energy of H is 13.6 eV, one needs photon energies above 6.7 eV (1 < 190 nm), which can be produced by frequency doubling of UV lasers in gases or metal vapors [574, 575]. 

Here, the stepwise excitation is, in particular, very helpful since only those Rydberg levels are excited that are connected by allowed electric dipole transitions to the known intermediate level, populated by the pump. This shall be illustrated by the example of a relatively simple molecule, the lithium dimer Li2 [581, 582]. When the pump laser selectively populates a level in the B Flu state, all... 

Fig. 5.42 Triple-resonance spectroscopy (a) population of high vibrational levels in the ground state by stimulated emission pumping and absorption of a third laser radiation, resulting in the population of levels just below and above the dissociation energy of the excited A Eu state of Na2 [617] (b) stepwise excitation of Rydberg levels by OODR and microwave-induced transitions to neighboring levels...

B. J. Dalton, Cascade Zeeman quantum beats produced by stepwise excitation using broad-line laser pulses. J. Rhys. B 20, 251, 267 (1987)... 

We first report on a two-laser photoionization experiment of benzene where the stepwise excitation was accomplished by frequency doubled dye lasers. The first laser pumped the molecule to a selected vibronic level of its first excited siglet state from where it was ionized... 

In order to alleviate the problem of laser wavelengths and non-state selectivity, stepwise excitation/ ionization via a resonant intermediate state has been introduced, termed RIS, or more generally REMPP, the minimum number of photons required is two (see Section 9.2 for a more extensive discussion of REMPI schemes). For the process to work as desired. 

Fig. 7.37a,b. Level schemes for Doppler-free three-photon spectroscopy of the Na atom (a) stepwise excitation of Rydberg states (b) Raman-type process shown for the example of the 3S3P excitation of the Na atom. Laser 1 is tuned while L2 is kept 30 GHz below the Na D line [7.64]... 

The second OODR scheme (Fig. 10.14b) represents stepwise excitation of high-lying levels via a common intermediate level 2), which is the upper level of the pump transition but the lower level of the probe transition. 

Here, the stepwise excitation is, in particular, very helpful since only those Rydberg levels are excited that are connected by allowed electric dipole transitions to the known intermediate level, populated by the pump. This shall be illustrated by the example of a relatively simple molecule, the lithium dimer Li2 [10.71,10.72]. When the pump laser selectively populates a level Jj ) in the Ylu state, all levels (u, 7 = 7 dz 1 or 7 = 7p in the Rydberg states ns E with = 0 and nd Delta, nd Tl, or nd S with = 2 and X = 2, 1, 0 are accessible by probe laser transitions with A7 = +1 (R-lines), A7 = 0 (Q-lines), or A7 = — 1 (P-lines). This is shown in (Fig. 10.25), where all possible transitions from the two A components of the B Ylu state with parity — 1 and +1 into the different Rydberg states are compiled. 

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