Pulse microwave

A microwave pulse from a tunable oscillator is injected into the cavity by an anteima, and creates a coherent superposition of rotational states. In the absence of collisions, this superposition emits a free-mduction decay signal, which is detected with an anteima-coupled microwave mixer similar to those used in molecular astrophysics. The data are collected in the time domain and Fourier transfomied to yield the spectrum whose bandwidth is detemimed by the quality factor of the cavity. Hence, such instruments are called Fourier transfomi microwave (FTMW) spectrometers (or Flygare-Balle spectrometers, after the inventors). FTMW instruments are extraordinarily sensitive, and can be used to examine a wide range of stable molecules as well as highly transient or reactive species such as hydrogen-bonded or refractory clusters [29, 30]. 

Figure Bl.16.20. FTEPR spectra of photogenerated DQ m TXlOO solution for delay times between laser excitation of ZnTPPS and microwave pulse ranging from 20 ns to 11 ps. The central hyperfme line (M= 0) is at s - 4.5 MHz. Reprinted from [63].

An alternative approach to obtaining microwave spectroscopy is Fourier transfonn microwave (FTMW) spectroscopy in a molecular beam [10], This may be considered as the microwave analogue of Fourier transfonn NMR spectroscopy. The molecular beam passes into a Fabry-Perot cavity, where it is subjected to a short microwave pulse (of a few milliseconds duration). This creates a macroscopic polarization of the molecules. After the microwave pulse, the time-domain signal due to coherent emission by the polarized molecules is detected and Fourier transfonned to obtain the microwave spectmm. 

HYSCORE, is a 2D four-pulse ESEEM technique which provides correlation between nuclear frequencies originating from different electron manifolds. The sequence of four microwave pulses is tx/2—x—tx/2—/tx— t2-nl2-x-echo where the echo amplitude is measured as a function of tx and t2 at fixed x. The a-proton anisotropic couplings can be detected by this technique (Konovalova et al. 2001a, Focsan et al. 2008). 

In this experimental system, it was necessary to trigger the laser pulse, the microwave pulse, and field modulation at the right timing, then to... 

The recent advent of the ability to apply short and very intense microwave pulses to samples and detect the fast response to the excitation has made it... 

The first experimental measurements of the time dependence of the hydrated electron yield were due to Wolff et al. (1973) and Hunt et al. (1973). They used the stroboscopic pulse radiolysis (SPR) technique, which allowed them to interpret the yield during the interval (30-350 ps) between fine structures of the microwave pulse envelope (1-10 ns). These observations were quickly supported by the work of Jonah et al. (1973), who used the subharmonic pre-buncher technique to generate very short pulses of 50-ps duration. Allowing... 

In the cross modulation experiments (Mentzoni and Row, 1963 Mentzoni and Rao, 1965), an electron plasma is briefly heated by a microwave pulse while a weak microwave signal probes the mean electron energy. Assuming no electron loss and insignificant ambient gas heating, these authors derived the following equation for the relaxation of electron Maxwellian temperature T.toward the ambient temperature T ... 

No specific recommendations can be given about the optimum reaction time. As speeding up reactions is a key motive for employing microwave irradiation, the reaction should be expected to reach completion within a few minutes. On the other hand, a reaction should be run until full conversion of the substrates is achieved. In general, if a microwave reaction under sealed-vessel conditions is not completed within 60 min then it needs further reviewing of the reaction conditions (solvent, catalyst, molar ratios). The reported record for the longest microwave-mediated reaction is 22 h for a copper-catalyzed N-arylation (see Scheme 6.63). The shortest ever published microwave reaction requires a microwave pulse of 6 s to reach completion (ultra-fast carbonylation chemistry see Scheme 6.49). 

Microwave-induced, catalytic gas-phase reactions have primary been pursued by Wan [63, 64], Wan et al. [65] have used pulsed-microwave radiation (millisecond high-energy pulses) to study the reaction of methane in the absence of oxygen. The reaction was performed by use of a series of nickel catalysts. The structure of the products seemed to be function of both the catalyst and the power and frequency of microwave pulses. A Ni/Si02 catalyst has been reported to produce 93% ethyne, whereas under the same irradiation conditions a Ni powder catalyst produced 83% ethene and 8.5 % ethane, but no ethyne. 

The two techniques, ENDOR and ESE envelope modulation, supplement each other. ESE envelope modulation seems to be more sensitive in detecting nuclear transitions at very low frequencies but is limited in the frequency range by yeB , where ye denotes the gyromagnetic ratio of the electron and Bj the microwave pulse amplitude. ENDOR, whose sensitivity increases with frequency, suffers on the other hand from the small transition probability at low frequencies. 

ESE-ENDOR. In ESE-ENDOR, introduced by Mims1181, a rf pulse in a three-pulse electron spin echo experiment is applied during the time interval between the second and third microwave pulse. The ENDOR spectrum is obtained by monitoring the decrease... 

ESEEM is a pulsed EPR technique which is complementary to both conventional EPR and ENDOR spectroscopy(74.75). In the ESEEM experiment, one selects a field (effective g value) in the EPR spectrum and through a sequence of microwave pulses generates a spin echo whose intensity is monitored as a function of the delay time between the pulses. This resulting echo envelope decay pattern is amplitude modulated due to the magnetic interaction of nuclear spins that are coupled to the electron spin. Cosine Fourier transformation of this envelope yields an ENDOR-like spectrum from which nuclear hyperfine and quadrupole splittings can be determined. 

For example, a low critical current density has been shown useful in using BaPb B Og as a microwave switch. The transmission of 2.8 GHz microwaves through a polycrystalline thin film was switched on in less than 30 ns via a high current pulse (107), resulting in short microwave pulses. 

Minami, K. Saeki, K., Kubo, H., Ohtsuka, M. Awano, M. and Takai, H., Quick Extraction of Microwave Pulses from a Cavity by a Superconducting Polycrystalline BaPbj xBixOa Thin Film. J. Appl. Phys.62(5) l902 (1987). 

Rapid stepping of the magnetic field, instead of using a second microwave frequency, has been used to measure interspin distances of the order of 20 A at X-band.28 A microwave pulse, called the pump pulse, is synchronized with the field step and occurs between the second and third pulses of a stimulated echo sequence. The effects of nuclear ESEEM were removed by dividing data obtained with a pump pulse (at the stepped magnetic field) by data obtained without a pump pulse. 

The second approach is to use thermal beams of alkali atoms as shown in Fig. 10.2.4 A beam of alkali atoms passes into a microwave cavity where the atoms are excited by pulsed dye lasers to a Rydberg state. A1 /zs pulse of microwave power is then injected into the cavity. After the microwave pulse a high voltage pulse is applied to the septum, or plate, inside the cavity to analyze the final states after interaction with the microwaves. By adjusting the voltage pulse it is possible to detect separately atoms which have and have not been ionized or to analyze by selective field ionization the final states of atoms which have made transitions to other bound states. 

The experiment is done using the apparatus shown in Fig. 10.2. Two tunable dye lasers are used to excite K atoms in a beam to the (n + 2)s state. A static field can be applied to vary the separation between the 18s and (16,k) states. The atoms are exposed to a microwave pulse, and subsequently to a field ionization pulse which ionizes atoms in the (n,k) Stark state, but not those in the (n + 2)s state. The (n,k) field ionization signal is then monitored as either the microwave field amplitude or the static field is swept over many shots of the laser. To the extent that both Stark shifts are linear, the static field alters the energy separation between the two states, but not their wavefunctions. 

Now imagine an experiment with states 1 and 2 analogous to the K experiments. Initially the atoms are in state 2 in a static field, the microwave pulse is applied, and afterwards the atoms can be in state 1 or state 2. During the microwave pulse we must use the full Hamiltonian of Eqs. (10.4). We can no longer represent the wavefunction by Eq. (10.5), but rather by... 

Some other experimental methods also require brief discussion here. The technique of microwave-pulse flash-spectroscopy is similar to that of flash photolysis, except that excitation is achieved by means of a powerful single pulse of microwave radiation from a magnetron19. The gas is contained in a quartz reaction vessel placed along the axis of a cylindrical cavity, tuned to the frequency... 

There are three widely accepted routes by which bone-conducted sound stimulates the cochlea. These are the compres-sional, inertial and osseotympanic theories of bone conduction (12). Compressional bone conduction implies that the cochlear shell is compressed slightly in response of the pressure variation caused by a sound. Inertial bone conduction alludes to a relative motion between the ossicular chain and the temporal bone for low frequency vibrations. The osseotympanic theory denotes a mechanism by which relative movement of the skull, with respect to the mandible, sets up pressure variation in the air present in the auditory meatus. Since perception of microwave pulses are correlated with the capacity to hear high-frequency sound, it rules out inertial or osseotympanic bone conduction as potential mechanisms for microwave acoustic effect. 

A vast amount of electrophysiological evidence has accumulated over the past several years demonstrating that auditory responses to microwave pulses are similar to those evoked by conventional acoustic pulses (6,7, U, 13,1 A, 15). Furthermore, they show that microwave acoustic effect is mediated by an electromechanical interaction which is initiated outside or at the cochlea. An alternative hypothesis involves direct stimulation of the cochlear nerve or neurons at higher levels along the auditory pathway. This latter mechanism is probably not at work as I shall presently demonstrate. 

This interpretation finds support in systematic studies of responses from brainstem nuclei following successive coagulative lesion production in the auditory loci (1 7). The effect of brainstem lesions on electrical potentials recorded from the superior olivary (SO) nucleus in response to microwave pulse stimulation is shown in Figure 3. Lesions in proximal nuclei (inferior colliculus, IC and lateral lemniscus, LL) had negligible influence on the response recorded from superior olivary nucleus. The response, however, disappeared after a lesion was made in its nucleus, thus, confirming the peripheral nature of the primary site of transduction. 

Figure 3. Effects of brainstem lesions on the microwave pulse-evoked auditory responses from the vertex of a cat ( 1)...

A mathematical analysis of pressure waves created by thermoelastic expansion of brain matter showed that the sound pressure required for human subjects to barely perceive microwave pulses is about the same as the known minimum audible sound pressure for bone conduction (1 3,27). The frequency of sound provides another line of evidence. It was shown that the fundamental frequency of sound is given by... 

---