Microwave transitions

The early MBER spectra were of two types first, pure rotational (microwave) transitions and, secondly, radiofrequency transitions between different hyperfine levels the latter were observable only for Ar-HCl, because... 

Figure 5.7 Eight components, with AT = 0 to 7 and separated by centrifugal distortion, of the 7 = 8 — 7 microwave transition of SiH3NCS...

Microwave spectroscopy is used for studyiag free radicals and ia gas analysis (30). Much laboratory work has been devoted to molecules of astrophysical iaterest (31). The technique is highly sensitive 10 mole may suffice for a spectmm. At microwave resolution, frequencies are so specific that a single line can unambiguously identify a component of a gas mixture. Tabulations of microwave transitions are available (32,33). Remote atmospheric sensing (34) is illustrated by the analysis of trace CIO, O, HO2, HCN, and N2O at the part per trillion level ia the stratosphere, usiag a ground-based millimeter-wave superheterodyne receiver at 260—280 GH2 (35). 

E. J. Lovas, Frequenciesfor Interstellar Molecular Microwave Transitions, Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, Md., 1996 on Internet at http //physics.nist.gov. 

Microwave spectroscopy The study of the interaction of microwave radiation with the rotational motion of a molecule. Microwave transitions are not restricted to molecules and can occur in atoms whenever the separation between the energy levels is in the microwave region of the spectrum. 

For the asymmetric top, K is not a good quantum number. Asymmetric-top A/ = 0 transitions correspond to lines of nonzero frequency, in contrast to the symmetric top. Moreover, it is possible for an asymmetric-top level with quantum number J — 1 to lie above a level with quantum number J (see Fig. 5.4) asymmetric-top microwave absorption lines with Ay= —1 are thus possible. Microwave transitions with Ay= -1, 0, and +1 are called / -, Q-, and / -branch lines, respectively, although these lines are interspersed among one another and do not really form branches as in the infrared spectrum. The dipole moment of an asymmetric top can be resolved into components along the three principal axes ... 

The lowest frequency microwave transitions of HC,2NM and DC,2N14 occur at 88,631 and 72,415 MHz, respectively. (These are for the ground vibrational state.) Use the formula of Problem 5.6 to calculate the bond distances in HCN. (Ignore zero-point vibrations.)... 

Since K is forbidden to change, a A7 = 0 transition with M changing leads to no absorption or emission of radiation.) Microwave transitions are studied in absorption. From (5.73), the frequencies of the microwave... 

Transitions due to a nonvanishing dipole-moment component da are called cr-type transitions. The selection rules for Kpr and Koh for the three kinds of asymmetric-top microwave transitions are... 

Measurements with Na+ ions of energies in the 29-590 e V range, corresponding to v/ve from 0.2 to 0.9, were compared to the diabatic SFI spectra of the Na 28f, 28g, and 28h states observed individually by driving resonant microwave transitions from the 28d state.9 These detailed comparisons show clearly that for high velocity, v/ve 0.9, 59% of the 28d— 28 cross section is to the 28f state, the dipole allowed transition. However, at lower values of v/ve, nondipole processes play a more important role. For example, at v/ve = 0.2, only 37% of the cross section is due to the 28d— 28f transition.9 At high velocities the process is predominantly a dipole M = 1 process, but at low velocities the dipole selection rule breaks down. 

A very simple method of observing the microwave transitions is to detect the fluorescence from only the final state. Gallagher et al.11,12 used this technique to detect one, two, and three photon Na transitions from the optically accessible nd states to the 3 5 states. The scheme used to detect the Na 16d— 16g... 

There are several considerations to bear in mind when using fluorescence detection. First, the approach is most useful when the photons to be detected have a vastly different wavelength than the exciting light and the most probable decay of the optically excited state, which need not be the same. Second, the branching ratio for the detected transition should be favorable. Third, the lifetimes of the initial and final state of the microwave transitions must be taken into account. If the microwaves are always on, at resonance, radiative decay occurs from the coupled pair of states. If the initial state of the microwave transition has a much... 

A way of doing microwave spectroscopy peculiar to the study of Rydberg atoms is to use selective field ionization to discriminate between the initial and final states of the microwave transition. An example of the application of this technique is the measurement of millimeter wave intervals between Na Rydberg states by Fabre et a/.13 using the arrangement shown in Fig. 16.5. 

Using two pulsed tunable dye lasers, Na atoms in a beam are excited to an optically accessible ns or ml state as they pass between two parallel plates. Subsequent to laser excitation the atoms are exposed to millimeter wave radiation from a backward wave oscillator for 2-5 [is, after which a high voltage ramp is applied to the lower plate to ionize selectively the initial and final states of the microwave transition. For example, if state A is optically excited and the microwaves induce the transition to the higher lying state B, atoms in B will ionize earlier in the field ramp, as shown in Fig. 16.5. The A-B resonance is observed by monitoring the field ionization signal from state B at fB of Fig. 16.5 as the microwave frequency is swept. 

In the method shown in Fig. 16.5, the excitation, microwave transition and selective ionization all occur in essentially the same place, since the thermal atoms do not move more than a few millimeters in a few microseconds. It is also possible... 

Fig. 16.5 (a) Schematic diagram of the experimental setup, (b) Energy diagram of the Na atom showing the levels populated by the stepwise laser excitation and the microwave transitions, (c) Sketch of the sequence of events experienced by the Na atoms (from... 

The second technique, selective field ionization, was used by Gentile et al.H to measure many intervals between low states of Ca. In their experiments the laser excitation, microwave transition, and detection were separated spatially as well as temporally, an approach similar to the one of Goy et al.9 described in Chapter 16. 

Most of the high precision spectroscopy of He Rydberg states has been done by microwave resonance, which is probably the best way of obtaining the zero field energies. Wing et a/.8-12 used a 30-1000pA/cm2 electron beam to bombard He gas at 10-5-10-2 Torr. As electron bombardment favors the production of low states, it is possible to detect A transitions driven by microwaves. The microwave power was square wave modulated at 40 Hz, and the optical emission from a specific Rydberg state was monitored. When microwave transitions occurred to or... 

A natural direction to improve the resolution of R measurements beyond the present level is to attempt the experiments at longer wavelengths, In a domain where the frequency of the radiation can be linked directly to the atomic time standard. A possible way is to measure R on microwave transitions involving what is called circular Rydberg atoms [4,73. 

In a second investigation of the microwave spectrum of 1,3,4-thiadiazole, 14N quadrupole coupling and centrifugal distortion for lines with J = 5 and / 50, respectively, are determined. These are compared with results obtained from 1,3,4-oxadiazoles and pyridazine. Comprehensive tables showing microwave transitions, the microwave spectrum and the a and n population from 14N quadrupole coupling data for 1,3,4-thiadiazole are also given (71JST(9)163> (see also Sections 4.01.3.2, 4.01.4.2.2.(i) and (iii), and 4.01 Table 2). 

There are several other important aspects of the experiment which should be mentioned. The waveguide cell is surrounded by a solenoid coil which can produce a magnetic field parallel to the ion beam direction the magnitude of this field (up to 50 G) is often sufficient to produce observable Zeeman splittings which greatly assist spectroscopic assignment, as we will see. ft is also possible to expose the molecular ion beam to two different microwave frequencies this so-called double resonance technique enables two different microwave transitions to be connected, if they share a... 

Figure 10.70. Lowest rotational levels and pure microwave transitions observed [202] for the CIO molecule in its 2 n ground state. The diagram is not drawn to scale, the fine-structure splitting being very much larger than the hyperfine or A-doublet splittings.

The chemical reaction produces CN in excited vibrational levels of the A state, and energy transfer from the v =10 level of the A state to the 0 = 0 level of the B state leads to strong fluorescent emission from the B state. The possibility of detecting microwave transitions in these excited states was investigated by Radford and Broida [3] and first realised experimentally by Evenson, Drum and Broida [4], We will discuss the nature... 

Figure 11.4. Energy levels and microwave transitions [4] involving the A 2Tl3/2 (v = 10) and B 2E+ (v = 0) excited electronic states of CN.

Figure 11.30. Energy level diagram and observed microwave transitions for CuO in the X2 n1/2 state [61]. The four A J = 0 yl-doublet transitions in the J = 3/2 level are shown on the right-hand side.

Figure 11.33. Energy level diagram and observed microwave transitions for TiO in its 3 A state.

Figure 11.34. Energy level diagram for the X4 states of CrN and MoN, and the observed microwave transitions [71].

Figure 11.36. The four lowest rotational levels of the X 2 A3/2 spin component of NiH, and the observed microwave transitions [76]. The energies are givenrelative to the lowest rotational level of the X2A5/2 component (with J = 5/2). The A-doublet and proton hyperfine splittings are exaggerated for the sake of clarity.

Figure 11.50. Energy level diagram (not to scale) showing the nuclear hyperfine structure of the HD+ 22,1 and 22,0 vibration-rotation levels (labelled with the G and G2 quantum numbers described in the text). The infrared transitions which give rise to the six lines shown in figure 11.49 (a) are shown on the left-hand side of the figure, and the four observed microwave transitions are shown on the right-hand side.

---