Spectrum band heads

Figure 13. Action spectrum of the linear He I Cl complex near the He + I Cl(By = 2) dissociation limit obtained by scanning the excitation laser through the ICl B—X, 2-0 region and monitoring the l Cl E—>X fluorescence induced by the temporally delayed probe laser, which was fixed on the l Cl E—B, 11-2 band head, (a). The transition energy is plotted relative to the I Cl B—X, 2-0 band origin, 17,664.08 cm . Panels (b), (c), and (d) are the rotational product state spectra obtained when fixing the excitation laser on the lines denoted with the corresponding panel letter. The probe laser was scanned through the ICl B—X, 11-2 region. Modified with permission from Ref. [51].

Fig. 1. Model Spectra re-binned to CRIRES Resolution To demonstrate the potential for precise isotopic abundance determination two representative sample absorption spectra, normalized to unity, are shown. They result from a radiative transfer calculation using a hydrostatic MARCS model atmosphere for 3400 K. MARCS stands for Model Atmosphere in a Radiative Convective Scheme the methodology is described in detail e.g. in [1] and references therein. The models are calculated with a spectral bin size corresponding to a Doppler velocity of 1 They are re-binned to the nominal CRIRES resolution (3 p), which even for the slowest rotators is sufficient to resolve absorption lines. The spectral range covers ss of the CRIRES detector-array and has been centered at the band-head of a 29 Si16 O overtone transition at 4029 nm. In both spectra the band-head is clearly visible between the forest of well-separated low- and high-j transitions of the common isotope. The lower spectrum is based on the telluric ratio of the isotopes 28Si/29Si/30Si (92.23 4.67 3.10) whereas the upper spectrum, offset by 0.4 in y-direction, has been calculated for a ratio of 96.00 2.00 2.00.

The last discovery of an isotope of an element in the second row of the periodic table was that of 15N. This is credited to R. Naude (1929). In the band spectrum of NO, he observed band heads for not only lsNieO but also 14N180 and 14N1tO on the basis of the expected isotope effect on the reduced mass of the molecule. 

The distinction between a truly continuous absorption spectrum and a banded absorption spectrum for diatomic molecules may be made by instruments of relatively low resolving power. Even though individual rotational lines are not resolved, a discrete spectrum will have sharp band heads and the appearance will in no way resemble the appearance of a continuum. 

Figure 4-1. (a) Fluorescence excitation spectrum of the Hg-H2 complex, (b) Action spectrum of the Hg-H2 complex the pump laser is varied while the probe laser sits on the band head of the 0-0 transition in Hg H(2n1/2 <- 2E+). In the 2=1 domain extending into the continuum seen in (a), one sees vibrational bands—the reaction is then slow for the fl = 0 excitation (in the red), no structure appears—indicating a fast reaction. 

Figure 4 shows the CARS spectrum of D2 gas scanned using a narrow band probe (u ) laser within the bright discharge region of an electrical discharge lamp. One can see the Q-branch band heads from both the v"=0 and v"=l levels. These spectra can be used to determine both the rotational and vibrational population distributions of D within the discharge. 

A portion of the medium-resolution spectrum of the visible 5-t— A iodine absorption spectrum with assignments for the overlapping progressions for u" = 0, 1, 2. The upper-state v values are indicated at the estimated band-head positions on the short-wavelength side of each transition the band maxima are at the top of the figure. 

A rough estimate of the rotational distribution can be obtained in the same manner as for BaF. A rotational temperature of 1000 K, independent of vibrational level, appears to be consistent with the observed fluorescence spectrum. The band head areas of the computer-synthesized spectra agree with the input data to 10 % for the stronger bands. 

In the region around k = 0.5, especially in the longer chains, several vibrations with frequencies close to (fi>rAM)max occur and the spectrum becomes congested into a band-head reminiscent of polyethylene. 

The limit beyond which the frequency shows no further increase is called the band head. It might, from its appearance in the spectrum, have been supposed to have special significance. Actually it has not, representing simply the frequency at which the terms in a quadratic happen to balance. According to the relative values of the different constants, the band head may lie on the side of longer or of shorter wave-lengths. The crowding of the lines in the one direction and... 

Instead of measuring the fluorescence intensity /fi(A.l), the excitation spectrum can also be monitored via resonant two-photon ionization (RTPI). This is illustrated by Fig. 4.7, which shows a RTPI spectrum of a band head of the Cs2 molecule. 

The reduction of Trot and Tyib results in a drastic simplification of the molecular absorption spectrum because only the lowest, still populated levels contribute to the absorption. Transitions from low rotational levels become stronger, those from higher rotational levels are nearly completely eliminated. Even complex spectra, where several bands may overlap at room temperature, reduce at sufficiently low rotational temperatures in cold beams to a few rotational lines for each band, which are grouped around the band head. This greatly facilitates their assignments and allows... 

The following free parameters were introduced (1) energy of the band head, (2) absolute intensity of the band, and (3) rotational parameter of the band. The last parameter was kept identical for all of the groups, i.e., the 5.28, 5.37 and 5.47 MeV groups. A fit to the measured spectrum was performed with the above parameters using the least squares method. The nonresonant part of the fission probability was taken into account as an exponential background. ... 

Fig. 9.7. Absorption spectrum of the (0-0) band head of the C <- X system of Cs2, monitored by resonant two-photon ionization spectroscopy in a cold collimated Ar beam seeded with cesium...

The fluorescence of the IF product molecules originating from the 6 11(0)+ -> band system was observed in the wavelength range 470-675 nm. The relative product vibrational population densities were determined from the measured fluorescence intensities of the individual bands, the relative laser intensities and the transition probabilities, as described in Ref. 12. At slow scanning speed the resolved rotational structure in the IF excitation spectrum was obtained, except in the vicinity of the band heads. It was analysed according to the procedure described below. 

To make quantitative statements about the product internal distribution a computer program is utilized to simulate the observed excitation spectrum [10]. As input for the calculations we estimate the relative vibrational and rotational populations. Each line is weighted by the population of the initial (v, J ) level, by the Franck-Condon factor and the rotational line strength of the pump transition. At each frequency, the program convolutes the lines with the laser bandwidth and power to produce a simulated spectrum such spectra are compared visually with the observed spectra and new estimates are made for the (v ,J") populations. Iteration of this process leads to the "best fit" as shown in the lower part of Fig. 3. For this calculated spectrum all vibrational states v" = 0...35 are equally populated as is shown in the insertion. The rotation, on the other hand, is described by a Boltzmann distribution with a "temperature" of 1200 K. With such low rotational energy no band heads are formed for v" < 5 in the Av = 0 sequence and for nearly all v" in the Av = +1 sequence (near 5550 A). 

---