Hot-band absorption

In addition to energetic considerations, however, there are other factors such as spin conservation that also determine the importance of various sets of products. As discussed in Chapter 3.A, since the ground state of 03 is a singlet, dissociation into either two singlet states (e.g., reaction (5)) or into two triplet states is expected to predominate. However, as discussed shortly, both hot-band absorption by rovibra-tionally excited 03 and by a spin-forbidden process are believed to contribute significantly to the atmospheric photochemistry of 03. 

It appears that there are two processes that contribute to this O( D) production. The first is so-called hot-band absorption by 03 in which the additional energy comes from internal vibrational and rotational energy (Michelsen et al., 1994), a phenomenon that is well established in the case of N02 photodissociation (see later). This is believed to be responsible for O( D)... 

Because of their low quantum defect, Er-doped transparent ceramics have interesting laser emissions, due to the infrared transitions of 1.5 pm Ii3/2 Iis/2 nd 3 pm " ln/2 li3/2> under resonant pumping into the emitting manifold. The strong absorption band of Er YAG at 1532.2 nm can be used for InP diode laser pumping of the " li3/2 level. This band collects the hot-band absorption transitions of 512Q) Ii3/2(1) at 1532.1 nm and %3/2(3) at 1532.3 nm. With... 

Fan et al. [98Fan] have measured fundamental and hot band absorption lines of Si CE, Si CE, SE CE, and Si CE using infrared velocity modulation spectroscopy (IR veloc. Mod.) between 630 and 700 cm . The following parameters have been determined (in cm if not otherwise indicated) ... 

Selective bond breaking has been demonstrated with HOD by first exciting the fourth overtone (local mode) of the OH bond and then photodissociating the molecule via the A X transition. The A <— X transition is red shifted (hot-band absorption) into the 240-270 nm region and the dissociation of the OH bond, relative to the OD bond, is enhanced by a factor of 15. This type of process is referred to as vibrationally mediated photodissociation and can be a very effective approach, provided the initial vibrational excitation remains localized in one chemical bond for a sufficient length of time to allow further excitation and dissociation. In the case of HOD it is clear that randomization of the vibrational energy is slower than the photodissociation step, and this further emphasizes the direct and impulsive nature of dissociation on the A Bi-state PES. 

A second, weaker (10 ) absorption system, known as the Huggins band (Figure 16.2 (bottom)), is observed on the long wavelength side of the Hartley band. Energetically, the main channel for the Hartley band, producing 02(a Ag) and 0( D2). is closed for X > 310 nm, but it is known that some 0( D2)( 8 per cent) is still produced through hot-band absorption and via a spin-forbidden channel, i.e. 

Also, it is my understanding that the initial transient observed by both yourself and Eisenthal and coworkers for 1-1 binaphtyl is associated with direct excitation of the relaxed conformational state by hot-band absorption to the next electronic state. Could you comment ... 

Some Cl product signals from hot band transitions due to the enhanced absorption of the vibrationally excited CH2CI radical are observed, particularly for excitation to the lowest l2Ai state in the wavelength region longer than 247nm.114,116 A recent theoretical study has... 

No vibrational analysis has been given for PtF6, but the absorption at 4.81 kK. seems most likely to correspond to a hot band (c.f. OsFg) and full d4 O calculations suggest that the A and A components of 3T1 g should not be separated by more than about 200 cm-1. On this... 

A third analysis of the UV absorption spectrum of borazine reported by Bernstein and Reilly is not in reement with Kaldor s assignments. Bernstein interprets the first vibrational progression in the same manner as Kaldor. However, for the second progression he reports a band at 2011 nm (not observed by Kaldor) which he interprets as a Vi hot band of aa" symmetry. He identifies the 197.5 nm origin of this band as the location of the Aj state. 

The X3 -—> B3X- transition is allowed and as seen in Figs. 4.2 and 4.3 results in an absorption in the 130-to 200-nrn region known as the Schumann-Runge system. The banded structure from about 175 to 200 nm corresponds to transitions from v" = 0 as well as v" = 1 (i.e., hot bands) of the ground X3S state to different vibrational levels of the upper state. 

Collision-induced emission. Emission spectra of ordinary atoms and molecules correspond to downward transitions, from an initial energy level higher than the final one, whereas absorption involves the inverse transition. Both exist in supermolecules as well and have recently been seen in shocktube studies. Emission spectra are generally much richer than absorption spectra and may include hot bands, which involve transitions between excited vibrational (or electronic) states [116]. 

The significance of collision-induced absorption for the planetary sciences is well established (Chapter 7) reviews and updates appeared in recent years [115, 165, 166, 169-173]. Numerous efforts are known to model experimental and theoretical spectra of the various hydrogen bands for the astrophysical applications [170, 174-181]. More recently, important applications of colhsional absorption in astrophysics were discovered in the cool and extremely dense stellar atmospheres of white dwarf stars [14, 43, 182-184], at temperatures from roughly 3000 to 6000 K. Under such conditions, large populations of vibra-tionally excited H2 molecules exist and collision-induced absorption extends well into the visible region of the spectrum and beyond. Numerous hot bands, high H2 overtone bands, and H2 rotovibrational sum and difference spectral bands due to simultaneous transitions that were never measured in the laboratory must be expected. Ab initio calculations of the collisional absorption processes in the dense atmospheres of such stars have yet to be provided so that the actual stellar emission spectra may be obtained more accurately than presently known. 

If the diatomic molecule has a low vibrational frequency, or if the gas is heated sufficiently, we will have substantial numbers of molecules in excited vibrational levels, and we will observe infrared absorption bands for which the initial vibrational level is not u = 0. Such bands are called hot bands. See Fig. 4.8. 

Hot Band.—Long wavelength contribution to an absorption band arising from absorption of radiation by molecules vibrationally excited but in their ground electronic state [e.g., the transition S(i> = ()) -So( ff = l)]. 

For transitions in absorption the Boltzmann distribution of molecules over the vibrational levels of the ground state implies that a majority of bands observed will emanate from the lowest, zero-point level transitions from higher levels will be weakened in proportion to the Boltzmann factor exp [ — Jiv/lcT], Hot bands arising from excited vibrational levels can be identified by studying the effect of temperature on the relative intensities. At ordinary temperatures the Boltzmann factor decreases approximately tenfold for each 500 cm-1 of vibrational... 

Early studies of the B1Ai(tc,tc ) <— X A n2) or S2 < S0 room-temperature absorption spectrum were carried out by Famworth and King [32], and then later in emission by Steer and co-workers [21]. Detailed analyses of the hot band structure were made by Judge and Moule [33], who located the origin for the system and identified several band progressions. In the course of recording the Sj S0. LIF excitation spectrum, Clouthier et al. [26] observed a UV emission... 

To conclude, regions B and C may show absorption-induced structures, especially thermally activated absorptions (hot bands). The diminution of this activated absorption causes the transition from region A to B in Fig. 2.9. Region B + C is a region of impurity, X-trap, or other spurious absorptions 41 it is unusable for quantitative analysis of the exciton phonon or polariton -phonon intrinsic relaxation mechanisms we investigate below. Therefore, our analysis will be concerned only with region A of the b- and a-polarized reflection spectra as the best candidates of a KK analysis. 

The 1 1 complex of water and hydrogen fluoride was also studied by Thomas 79), from 4000 to 400 cm-1. This is an experimentally difficult task in view of the low volatility of the H2O.HF complex. The analysis of the spectrum shows that the complex is coplanar, C2v. This splits the degeneracy of vb and vp the two bridge deformation vibrations which are degenerate in the linear or C3v complexes. Vj has some structure consisting of a sharp band at 3608 cm-1, a broader band split into two at 3623 and 3626 and a broad band at 3644 cm-1 followed by continuous absorption (Fig. 10). The free-associated separation is 354 cm-1 for H2O.HF while it is 420cm-1 for dimethylether. HF. (Arnold and Millen20. ) As in the previous cases the fine structure can be interpreted as a series of hot bands, (vt + n vp — n"vp). 

Figure 7 Polarized IR absorption spectra and their ratio measured 100 ps after photolysis of Mb13CO in D20. The left axis corresponds to the photolysis-induced absorbance changes, AA1- (thick lines) and AA11 (thin lines) the right axis corresponds to their ratio AA /AA11 (open circles). The ratio is plotted where the absorbance exceeds 25% of its maximum and has been corrected for fractional photolysis. The dashed lines correspond to the average A- and B-state ratios. The A- and B-state spectra were collected at 10.8% and 20% photolysis of Mb13CO, respectively. The background and hot band contributions to the B-state spectra have been removed. (Adapted from Ref. 51.)...

Although direct access to the (10000), (01000), and (00010) levels from the (00000) ground-state level by infrared absorption is thus rigorously forbidden by symmetry, access from molecules in the (00010) or (00001) levels can be symmetry allowed. For example F(OOOOl) X F(IOOOO) = n X 2 = n = r(/t ), and so the transition between these levels, termed a difference band, — v, is not formally forbidden. As can be seen in Fig. 1, the frequency (vi - v ) can be added to the fundamental frequency to give the evarf value of the (lOOOO)-(OOOOO) spacing. Similarly, the V2 and 3 difference bands are infrared-active and can be combined with and v, to deduce V2 and v, respectively. Such difference bands are detectable for acetylene but will of course have low intensity because they originate in excited levels that have a small Boltzmann population at room temperature. The intensity of such bands increases with temperature, hence they are also termed hot-band transitions. 

Other experimental and theoretical methods have been developed for the determination of the heat of sublimation of solid iodine these too are suitable for undergraduate laboratory experiments or variations on this experiment. Henderson and Robarts have employed a photometer incorporating a He-Ne gas laser, the beam from which (attenuated by a CUSO4 solution) has a wavelength of 632.8 nm, in a hot band near the long-wavelength toe of the absorption band shown in Fig. 3. Stafford has proposed a thermodynamic treatment in which a free-energy function ifef), related to entropy, is used in calculations based on the third law of thermodynamics. In this method either heat capacity data or spectroscopic data are used, and as in the present statistical mechanical treatment, the heat of sublimation can be obtained from a measurement of the vapor pressure at only one temperature. 

---