Spectral narrowing

Spectral narrowing simply means that an absorption in the spectrum is surprisingly narrow. In Chapter 6, we mentioned that some spin-forbidden transitions can be seen as very sharp absorptions in transition metal systans. The reason in that case is that the same spatial orbitals are occupied as in the ground state and therefore, the bond lengths are almost the same in the ground state and in the excited state. The Stokes shift is close to zero and therefore, there is no vibrational broadening. 

Narrowing of a more dynamic type occurs in laser emission where the emission with the highest intensity stimulates emissions with the same wavelength. These wavelengths are therefore intensified compared with missions with less intensity. The line is sharpened. 

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Above a pump energy of 20 pJ for the incident laser beam a strong spectral narrowing of the PL-spcclra of m-LPPP could be detected. The recorded spectra... 

G.J. Denton, N. Tessler, M.A. Slevens, R. H. Friend, Spectral narrowing in optically pumped polyO -phenylencvinylene) films, Adv. Mater. 1997, 9, 547. 

For this study, thin films were deposited onto glass substrates. The as-deposited films showed no spectral narrowing at any pump energy up to the damage threshold. Stimulated emission was observed only in annealed films. The spol-to-spot reproducibiliiy of the measured characteristics was good, and we could measure with excitation energies of up to 4 mJ (1.8 mm beam diameter) without visual damage of the illuminated spot. 

As discussed in [91], the shape of a static spectrum determines significantly the spectral transformation as frequency exchange increases. In particular, spectral narrowing will take place only if the second moment of the spectrum is finite. In our case... 

These results, first obtained by Kubo [188], permit us to show the general nature of spectral narrowing phenomena as arising from modulation which satisfies the demands of Eq. (4.72). 

Keilson-Storer kernel 17-19 Fourier transform 18 Gaussian distribution 18 impact theory 102. /-diffusion model 199 non-adiabatic relaxation 19-23 parameter T 22, 48 Q-branch band shape 116-22 Keilson-Storer model definition of kernel 201 general kinetic equation 118 one-dimensional 15 weak collision limit 108 kinetic equations 128 appendix 273-4 Markovian simplification 96 Kubo, spectral narrowing 152... 

The higher branched derivatives of the spiro compounds, 43 and 44 also exhibit spectral narrowing under pulsed excitation, with emission lines at 428 and 443 nm, and line widths of 3.2 and 3.9 nm, respectively (Fig. 3.24). For the same film thickness, the threshold is lower for 4-Spiro2 than for spiro-sexiphenyl despite the lower absorption, which is attributed to the higher luminescence quantum yield. 

Fig. 1 b shows the 2D-IR spectrum of the rhenium complex (4 mM in DMSO) in the electronic ground state. The 2D-IR spectrum is measured according to our double-resonance scheme [2,14], employing a spectrally narrow IR pump pulse and a broadband IR probe pulse... 

The guanine CO-stretch vibration is excited at three different pump frequencies with a spectrally narrow laser pulse (FWHM 12 cm 1). Transient-absorption spectra are measured using a systematically delayed, spectrally broad, probe pulse. A negative transient-absorption signal indicates a bleaching, whereas a positive signal reflects excited-state absorption. The overall decay of the signal is due to T relaxation. 

For 3T, a broad A band was observed that becomes spectrally narrower within the first picosecond. The maximum of A1 lies at X = 600 nm. Additionally, small positive values of AD at very small delay times, indicating a further transient absorption A0, were found. The transient behavior of 4T showed that the maximum of A was found at about X = 770 nm. It was found that the whole absorption band A decays simultaneously and single exponentially with x = 530 ps. Positive AD values due to an absorption A0 seem to appear weak at zero decay. 

Deviations from predicted relaxation behavior have been observed for large proteins (3 -7 ), polymers (8y9j and highly associated small molecules (10). Particularly prominent are observations of Ti field dependences and low NOE s within the so-called "extreme spectral narrowing region," where single correlation time models predict field independence of Tp and full NOE s. 

Dembele, S., Zhang, J., and Wen, J.X. Assessments of spectral narrow band and weighted-sum-of-gray-gases models for computational fluid dynamics simulations of pool fires. Numerical Heat Transfer Part B, 2005.48(3), 257-276. 

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