First optical spectrum

The first optical spectrum of allene by Sutcliffe and Walsh has been followed by measurements of Rabalais et aO that provide the spectrum from 4.78 to 10.2 eV. A moderately high resolution gas phase study from 6.2 to 10.7 eV is described by Iverson and Russell Fuke et al. also report the absorption and MCD spectra in both the gas phase and in perflurohexane solution from 6.2 to 8.0 eV. The absorption spectrum of allene is rather complex. There is weak structureless absorption below 6.45 eV. Four distinct absorption bands are observed between 6.54 and 9 eV of which the first band is weak with a maximum at 6.70 eV, and a strong broad absorption covers the range of 6.95-7.85 eV. Five distinct peaks of roughly the same intensity are present in the 8.02-8.38 eV region, followed by a strong transition around 8.57 eV. The absorption bands beyond 8.57 eV are fairly complicated and remain to be characterized. 

Irradiation of DMDAF in benzene solution at room temperature generates a transient intermediate that appears within the rise-time of the laser and decays by a pseudo first-order kinetic path with a half-life of 51 ns. The optical spectrum of the intermediate is essentially the same as that observed in the low temperature irradiation experiment. 

Diphenylmethylene is certainly the most exhaustively studied of the aromatic carbenes. Low temperature epr spectroscopy (Trozzolo et al., 1962) clearly established the ground state of this carbene as the triplet. The optical spectrum of the triplet was recorded first in a 1,1-diphenylethylene host crystal (Closs et al., 1966) and later in frozen solvents (Trozzolo and Gibbons, 1967). 

However, in benzene containing an excess of CO and in the absence of any other donor L, the existence of Fe(Deut-DME)(CO)2 (- -[75]) was indicated by its optical spectrum. Its formation from Fe(Deut-DME)CO has been attributed a formation constant K2 = 4 10 3 mol-11-1, while the binding of the first CO to Fe(Deut-DME) occurs with Kx = 5 104 mol-11-1 (27). The large difference of Kj and K2 are a quantitative expression of the thermodynamic 7r-acceptor tram effect. 

Since the photophoretic force depends on the electromagnetic absorption efficiency Q y , which is sensitive to wavelength, photophoretic force measurements can be used as a tool to study absorption spectroscopy. This was first recognized by Pope et al. (1979), who showed that the spectrum of the photophoretic force on a 10 foa diameter perylene crystallite agrees with the optical spectrum. This was accomplished by suspending a perylene particle in a Millikan chamber with electro-optic feedback control and measuring the photophoretic force as a function of the wavelength of the laser illumination. Improvements on the technique and additional data were obtained by Arnold and Amani (1980), and Arnold et al. (1980) provided further details of their photophoretic spectrometer. A photophoretic spectrum of a crystallite of cadmium sulfide reported by Arnold and Amani is presented in Fig. 11. 

From the photoelectron spectra of planar aromatics 1241 it was found that the position of the a-band relative to the p-band can be predicted from the first two IP s. When the difference AIP = IP2 — IPi > 0.7 eV, then the p-band is the first band in the optical spectrum when AIP < 0.5 eV the a-band is at longer wavelength. 

Figure 1. Schematic view of dynamical processes, associated time scales (see text), and observables. The first bottom panel displays a typical ionization cross-section, the second one a typical optical spectrum associated to Mie plasmon and the third one sketches a possible distribution of fragments. From [6].

Figure 4. A) Room-temperature optical spectrum of a single crystal of plastocyanin obtained with light incident on the (0,1,1) face and polarized parallel (solid line) and perpendicular (dashed line) to a (from Ref. 11). B) Gaussian resolution of the 35 K visible absorption spectrum of a plastocyanin film with suggested assignments the symbols ( ) represent the experimental absorption spectrum. Right plastocyanin unit cell projected on the (0,1,1) plane, showing the positions of the four symmetry-related Cu atoms at their first coordination shells.

One example of a concrete system where one observes optical spectrum caused by the Ai-E electronic transition is the N-V center in diamond. This center consists of a substitutional N atom and three nearest C atoms (one of the nearest C atoms is replaced by the vacancy) and it has a trigonal symmetry. The ZPL line at 637 nm of this center corresponds to the electronic transition between the triplet 3A and the 3E electronic states. In the standard model of this center the electronic states of the center come from the occupation and the splitting of the aj and t2 levels arising from three C radicals. The crystalline field of a trigonal symmetry splits the t2 level into a number of states including the ground (Aj) state and the first excited E-state (see, e.g. Refs. [17-25]). Our experimental study of the optical transition between the E and the Aj electronic states indeed showed the 7 3 dependence of the ZPL width at low temperatures. 

In this system with even number N of dimer units a orbitals HOMO and LUMO are nonbonding with zero overlap [9], Therefore, the photo-induced electron transition between these orbitals is forbidden. The first electron transition with lowest energy in optical spectrum of this system proceeds between HOMO and unoccupied molecular orbital next to LUMO [6]. Simple calculations based on formula (9) give the energy AEt of this transition at N 1 as... 

TDDFT methods have also been applied successfully to the description of the linear and nonlinear optical properties of heteroleptic sandwich complexes. The optical spectrum and the hyperpolarizability of Zr(OEP)(OEPz,) for which large first hyperpolarizabilities, /JSHG (SHG=second-harmonic generation) were measured in an electric field induced second-harmonic generation (EFISH) experiment [182], have been investigated by TDDFT methods [134]. The excitation energies and oscillator strengths calculated... 

In this paper, we address two aspects of this general problem. First, we discuss the problem of frequency standards in the optical spectrum. (An analogue in the microwave region of the spectrum is the cesium beam frequency standard.) If one or a few of these reference frequencies can be accurately calibrated (perhaps by a frequency synthesis chain- -) then it may be possible to compare optical spectra to these standards. As an example of the precision that might be achieved, we discuss only optical standards based on stored ions. Second, we discuss the problem of frequency comparison of unknown frequencies to the standards. Here we primarily restrict discussion to generation of wideband frequency "combs". 

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