Molecular masers

The ro-vibronic spectrum of molecules and the electronic transitions in atoms are only part of the whole story of transitions used in astronomy. Whenever there is a separation between energy levels within a particular target atom or molecule there is always a photon energy that corresponds to this energy separation and hence a probability of a transition. Astronomy has an additional advantage in that selection rules never completely forbid a transition, they just make it very unlikely. In the laboratory the transition has to occur during the timescale of the experiment, whereas in space the transition has to have occurred within the last 15 Gyr and as such can be almost forbidden. Astronomers have identified exotic transitions deep within molecules or atoms to assist in their identification and we are going to look at some of the important ones, the first of which is the maser. 

Radiative decay. The excited state can spontaneously decay and emit a photon. 

The rate at which this happens determines a radiative lifetime determined by 

Collisional deactivation. Collisions with other molecules can stimulate the relaxation to the ground state. There are no selection rules for collisionally induced transitions. 

Stimulated emission. The upper state can also decay by stimulated emission controlled by the Einstein B coefficient and the intensity of photons present of the same frequency. 

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Aside from molecular maser emission of OH, HpO and CH OH there exist several additional indicators of star formation in molecular clouds ... 

For molecular masers, it can be assumed that the two masing levels have the same statistical weight. Thus, B 2 = B21 = B. The number of levels involved in the particular population inversion is small. This implies strong radiative coupling between states which, for some reason, are selectively pumped by the external sources of... 

The shape of the maser curve not only depends on the rubber compound, but also on the surface on which it slides. On dry, clean polished glass the friction master curve for gum rubbers rises from very small values at low log ajv to a maximum which may reach friction coefficients of more than 3 and falls at high log ajv to values which are normally associated with hard materials, i.e., 0.3 shown for an ABR gum compound in Figure 26.2. If the position of the maximum on the log a-fV axis for different gum rubbers is compared with that of their maximum log E frequency curves, a constant length A = 6 X 10 m results which is of molecular dimension, indicating that this is an adhesion process [10]. 

Also, the laser or maser measures time very precisely. The hydrogen maser, for example, can measure times with a precision of about 10 15. It can also measure short times - as short as 10 15 seconds. So we can see atomic and molecular phenomena occurring on very short time scales. 

Abstract A molecular line search in the range between 85 and 89 GHz has been performed in the circumstellar envelopes of 11 evolved stars. Emissions of 29SiO J=2-l,28SiO J=2-l, HCN J=l-0, H13CN J=l-0, HC5 N J=33-32, HCO+ J=l-0 transitions and other transitions of C2 H, C4 H, and C3 N have been observed in 11 stars. We have detected the ground state 29SiO J=2-l maser in several stars. We have also detected HCN emission in VY CMa. A narrow H13CN spike feature near the central velocity has been found in the spectrum of CRL 2688. 

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. 

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.

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... 

From the preceding discussions it is evident that at least four different temperatures have to be considered which under laboratory conditions are all equal the excitation temperature Tex of the molecule, defined by the relative populations of the levels, the kinetic temperature Tk, corresponding to the Maxwellian velocity distribution of the gas particles, the radiation temperature Traa, assuming a (in some cases diluted) black body radiation distribution, and the grain temperature 7, . With no thermodynamic equilibrium established, as is common in interstellar space, none of these temperatures are equal. These non-equilibium conditions are likely to be caused in part by the delicate balance between the various mechanisms of excitation and de-excitation of molecular energy levels by particle collisions and radiative transitions, and in part by the molecule formation process itself. Table 7 summarizes some of the known distribution anomalies. The non-equilibrium between para- and ortho-ammonia, the very low temperature of formaldehyde, and the interstellar OH and H20 masers are some of the more spectacular examples. 

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... 

Most molecular species can be expected to possess a nuclear structure, although for some, e.g., hydrogen bonded species or ammonia, the issue is not so obvious. The ammonia-maser transition, for example, is thought to be a transition between strange molecular (pure) states that do not admit a nuclear structure. 

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... 

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). 

Cen- Molecular, Wave Invention of the maser Discovery of Raman effect ... 

In one laboratory exercise, students use a radio telescope to measure the emission spectra of selected molecules in astronomical sources, such as dense clouds and old stars. Molecular hyperfme structure, classical spectral patterns of linear and symmetric top species, and maser action are investigated. Another laboratory exercise was developed where students measure the rotational spectra of HCO, HCCCN, and CH3CN in the laboratory, and determine their unique spectroscopic properties. They then use their measurements to identify these molecules in interstellar gas. 

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, ... 

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