Rotational emission spectra

Table 17.1. Observed interstellar and circumstellar molecules The species listed are observed by their rotational emission spectra unless...

Seto JY, Morbi Z, Harron FC et al (1999) Vibration-rotation emission spectra and combined isotopomer analyses for the coinage metal hydrides CuH CuD, AgH AgD, and AuH AuD. J Chem Phys 110 11756-11767... 

Mann DE, Thrush BA, Lide JDR et al (1961) Spectroscopy of fluorine flames. I. Hydrogen-fluorine flame and the vibration-rotation emission spectrum of HF. J Chem Phys 34 420-431... 

By obtaining values for B in various vibrational states within the ground electronic state (usually from an emission spectrum) or an excited electronic state (usually from an absorption spectrum) the vibration-rotation interaction constant a and, more importantly, B may be obtained, from Equation (7.92), for that electronic state. From B the value of for that state easily follows. 

The emission spectrum observed by high resolution spectroscopy for the A - X vibrational bands [4] has been very well reproduced theoretically for several low-lying vibrational quantum numbers and the spectrum for the A - A n vibrational bands has been theoretically derived for low vibrational quantum numbers to be subjected to further experimental analysis [8]. Related Franck-Condon factors for the latter and former transition bands [8] have also been derived and compared favourably with semi-empirical calculations [25] performed for the former transition bands. Pure rotational, vibrationm and rovibrational transitions appear to be the largest for the X ground state followed by those... 

Optical emision spectra nowadays are simply measured using a fiber optic cable that directs the plasma light to a monochromator, which is coupled to a photodetector. By rotating the prism in the monochromator a wavelength scan of the emitted light can be obtained. Alternatively, an optical multichannel analyzer can be used to record (parts of) an emission spectrum simultaneously, allowing for much faster acquisition. A spectrometer resolution of about 0.1 nm is needed to identify species. 

In the ideal case of free Eu + ions, we first must observe that the components of the electric dipole moment, e x, y, z), belong to the irreducible representation in the full rotation group. This can be seen, for instance, from the character table of group 0 (Table 7.4), where the dipole moment operator transforms as the T representation, which corresponds to in the full rotation group (Table 7.5). Since Z)° x Z) = Z) only the Dq -> Fi transition would be allowed at electric dipole order. This is, of course, the well known selection rule A.I = 0, 1 (except for / = 0 / = 0) from quantum mechanics. Thus, the emission spectrum of free Eu + ions would consist of a single Dq Ei transition, as indicated by an arrow in Figure 7.7 and sketched in Figure 7.8. 

Fig. 7.3. Upper figure Emission spectrum of Jupiter in the far infrared two diffuse, dark fringes are seen at the H2 Sb(0) and Sb(l) rotational transition frequencies, caused by collision-induced absorption in the upper, cool regions. The lower figure presents an enlarged portion which shows the dimer structures near the So(0) transition frequency [150].

The use of tunable lasers as sources in electronic absorption and emission spectroscopy has made possible a very considerable increase in resolution and precision. Electronic spectra are often difficult to analyze because of the many transitions involved. However, with a tunable laser source, one can tune the laser frequency to a specific absorption frequency of the molecule under study and thus populate a single excited electronic vibration-rotation energy level the resulting fluorescence emission spectrum is then simple, and easy to analyze. 

A breaking off of the branches has been observed in the emission spectrum of HNO by Clement and Ramsay25 and, correspondingly, higher rotational lines have been found to be diffuse in absorption.28 These observations lead to an upper limit of 2.11 eV for D(H—NO). 

In this case, as with all other hydrogen halide lasers, only P branch transitions are observed, indicating that only partial inversion is attained. The vibrational transitions observed are 1 - 0 and 2 -+ 1. There is a definite threshold flash energy, below which no laser action is observed because the chain decomposition is not fast enough. The development in time of the emission spectrum was observed and discussed in terms of rotational relaxation. 

In 1913 Bohr amalgamated classical and quantum mechanics in explaining the observation of not only the Balmer series but also the Lyman, Paschen, Brackett, Pfund, etc., series in the hydrogen atom emission spectrum, illustrated in Figure 1.1. Bohr assumed empirically that the electron can move only in specific circular orbits around the nucleus and that the angular momentum pe for an angle of rotation 9 is given by... 

The a3 n state of CO was first identified through its ultraviolet emission spectrum to the ground state, producing what are now known as the Cameron bands [160, 161, 162], Its radioffequency spectrum was then described by Klemperer and his colleagues in a classic series of molecular beam electric resonance experiments. Its microwave rotational spectrum was measured by Saykally, Dixon, Anderson, Szanto and Woods [163], and the far-infrared laser magnetic resonance spectrum was recorded by Saykally, Evenson, Comben and Brown [164], In the infrared region both electronic... 

An example of an alternative use of FT technology in the UV/VIS is our work on the X2Z" B2Z+ emission spectrum of jet-cooled CN [21], These experiments were made possible by the development of the corona-excited supersonic expansion source by Engelking [20]. The Engelking source creates radicals in a continuous discharge, followed by immediate cooling in the expansion. A high number density of rotationally and translationally cold radicals in excited electronic and vibrational states is produced. As a result, excited vibronic states of reactive species can be studied with a minimum of rotational congestion. 

The X — B emission spectrum of CN measured at 0.25 cm-1 resolution is shown in Figure 19. The CN radical in its B state was produced by coexpanding 100 torr of acetonitrile (CH3CN) with 1 atm of helium in the corona discharge source. The spectrum includes both the 0-0 and 1-1 transitions. An analysis of the rotational distributions in both the v = 0 and 1 levels revealed a Boltzmann temperature of... 

Figure 19. Jet-cooled emission spectrum of the X <- B transition of CN measured at 0.25 cm-1 resolution. The expansion conditions were 100 Torr of acetonitrile seeded in 1 atm of helium. Both the 0-0 and 1-1 transitions are shown. The arrows indicate the small perturbations due to A state rotational levels (see text).

Electronic Spectrum. Acetone is the simplest ketone and thus has been one of the most thoroughly studied molecules. The it n absorption spectrum extends from 350 nm and reaches a maximum near 270 nm (125,175). There is some structure observable below 295 nm, but no vibrational and rotational analysis has been possible. The fluorescence emission spectrum starts at about 380 nm and continues to longer wavelengths (149). The overlap between the absorption and the fluorescence spectra is very poor, and the 0-0 band has been estimated to be at - 330 nm (87 kcal/mol). The absorption spectra, emission spectra, and quantum yields of fluorescence of acetone and its symmetrically methylated derivatives in the gas phase havbe been summarized recently (101). The total fluorescence quantum yield from vibrationally relaxed acetone has been measured to be 2.1 x 10 j (105,106), and the measurements for other ketones and aldehydes are based on this fluorescence standard. The phosphorescence quantum yield is -0.019 at 313 nm (105). 

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