Molecular beam maser

Figure 8.46. Principles of a molecular beam maser spectrometer. Population inversion of the A -doublet levels is produced in the quadrupole field, leading to enhanced stimulated emission in the microwave cavity.

Although molecular inversion is a phenomenon which theoretically can occur in any nonplanar molecule, from the point of view of vibration-rotation spectroscopy inversion is of significance for relatively few molecules. Nevertheless, molecular inversion is ail interesting and important large-amplitude molecular motion. Inversion has pronounced effects on the spectra of certain molecules experimental as well as theoretical studies of these effects became an important part of the history of molecular spectroscopy. The results of these studies found also important applications, the best-known example being the celebrated NH3 molecular beam maser. 

Ammonia was the first molecule for which the effect of the molecular inversion was studied experimentally and theoretically. Inversion in ammonia was subsequently found to be so important that this molecule played an important role in the history of molecular spectroscopy. Let us recall, for example that microwave spectroscopy started its era with the measurements " of the frequencies of transitions between the energy levels in the ground vibronic state of NH3 split by the inversion effect. Furthermore, the first proposal and realization of a molecular beam maser in 1955 was based on the inversion splittings of the energy levels in NH3. The Nobel Prize which Townes, Basov and Prochorov were awarded in 1964 clearly shows how important this discovery was. Another example of the role which the inversion of ammonia played in the extension of human knowledge is the discovery of NH3 in the interstellar space by Cheung and his co-workers in 1968, by measuring the... 

More recently, improved resolution (between 5 and 20 times better) was obtained by examining several of the rotational transitions in a molecular beam maser spectrometer [2145]. This gave more precise values of the F spin-rotation constants along the principal inertial axes (A/gj = -19.77 kHz = -13.46 kHz = -7.80 kHz) [2145], and the F... 

A number of halogenomethanes have been subjected to other forms of molecular spectroscopy. High-resolution Stark spectra of several transitions of the V3 band of CH3F have been studied by means of a CO2 laser measurements of the hyperfine structure on certain rotational transitions in CH2F2 have been made using a molecular beam maser spectrometer the millimetre-wave spectrum of ground-state CDCla and the microwave spectrum of CD3I in excited vibrational states have also been observed. 

Theoretical analysis of the vibrational spectra of FgCO, CI2CO, and BraCO and the electronic spectra of (CN)2CO has been attempted the hyperfine structure on certain rotational transitions in F2CO has been observed using a molecular beam maser spectrometer, ... 

I have chosen to include only those lines whose production requires what may be termed standard techniques . Electrically excited and optically pumped lasers are included, except those involving multi-photon systems, tunable pump lasers, or stark-shifted [1.16] far-infrared lasing. These are all useful techniques but beyond the purposes of this book. Gas-dynamic lasers [1.17], chemical lasers and molecular beam masers [1.18] are also excluded. 

F.C. de Lucia, W. Gordy Millimeter and Submillimeter Wave Molecular Beam Masers , in Proc. Symp. Submill. Waves, Microwave Research Institute Symposia Series Vol. XX, ed. by J. Fox (Polytechnic Press, New York 1970)... 

What has not been included As mentioned, the lines of CO2 and N2O lasers themselves are not included. They are very numerous, and are already catalogued in a number of references (see Chapter 5). TEA-laser pulsed lines, or any others with pulse lengths less than 1 /iS, are not included. Stark-shifted and multiple photon lines are excluded, as are molecular beam masers and lasers relying on chemical reactions for their excitation. 

Inversion splitting of the vibrational spectrum of ammonia has been used to create the first molecular microwave amplifier (maser) [86, 87]. The inversion population in the ammonia maser is achieved by transmission of the molecular beam through a non-homogeneous electric field. Ammonia molecules in symmetric and antisymmetric states interact with the electric field in different ways and they are therefore separated in this field. They are then directed to the resonator. 

The first successful application of molecular beam electric resonance to the study of a short-lived free radical was achieved by Meerts and Dymanus [142] in their study of OH. They were also able to report spectra of OD, SH and SD. Their electric resonance instrument was conventional except for a specially designed free radical source, in which OH radicals were produced by mixing H atoms, formed from a microwave discharge in H2, with N02 (or H2S in the case of SH radicals). In table 8.24 we present a complete A-doublet data set for OH, including the sets determined by Meerts and Dymanus, with J = 3/2 to 11/2 for the 2n3/2 state, and 1/2 to 9/2 for the 2ni/2 state. Notice that, for the lowest rotational level (7 = 3/2 in 2n3/2), the accuracy of the data is higher. These transitions were observed by ter Meulen and Dymanus [143], not by electric resonance methods, but by beam maser spectroscopy, with the intention of providing particularly accurate data for astronomical purposes. This is the moment for a small diversion into the world of beam maser spectroscopy. It has been applied to a large number of polyatomic molecules, but apparently OH is the only diatomic molecule to be studied by this method. 

J = 3/2, 5/2 and 7/2 levels of both fine-structure states. Also shown are the /l-doublet transitions observed, first by Dousmanis, Sanders and Townes [4], and subsequently by ter Meulen and Dymanus [165] andMeertsandDymanus [166]. The later studies [166] used molecular beam electric resonance methods which were described in chapter 8, and the most accurate laboratory measurements of transitions within the lowest rotational level were those of ter Meulen and Dymanus [165] using a beam maser spectrometer, also described in chapter 8. In the years following these field-free experiments, attention... 

The resulting spectrum represents a weighted average over the rotational states and a careful analysis of it yields the nuclear quadrupole coupling constant. Molecular beam electric resonance is complementary to pure rotational spectroscopy since transitions between the Stark levels of the rotational states (AJ = 0) are observed (sometimes AJ= 1 transitions are also studied). Specially constructed maser spectrometers55 that can detect transitions of rotationally selected molecules have been used to determine very small coupling constants, such as those for deuterium compounds. Again, molecular beam resonance is currently limited to the study of small molecules. 

In 1951, Purcell, Pound and Ramsey [21] did some NMR experiments with inverted populations of the nuclear spin systems in LiF and noted that the spin systems were at negative absolute temperatures and that they were intrinsic amplifiers rather than absorbers. The first suggestions to use systems with inverted populations as practical amplifiers or oscillators were made independently in the early 1950 s by Townes [15,22], Weber [15] and Bassov and Prokhorov [15]. The first such amplifier was a molecular beam apparatus operating on the NH3 inversion states and built by Gordon, Zeiger and Townes [22] and was called a microwave amplifier by stimulated emission of radiation (MASER). 

Verhoevan, J., and A. Dymanus, Magnetic properties and molecular quadrupole tensor of the water molecule by beam-maser Zeeman spectroscopy. J. Chem. Phys., 1970. 52 3222-3233. 

J. Verhoeven and A. Dymanus, ]. Chem. Phys., 52, 3222 (1970). Magnetic Properties and Molecular Quadrupole Tensor of the Water Molecule by Beam-Maser Zeeman Spectroscopy. 

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