Molecule excited

The excited states produced may or may not lead to chemical change. Their reactions may be summarised as follows 

It will be convenient to review firstly the evidence for the production of specific excited states in radiation chemistry and then to examine the details of some of their reactions. 

The second-order rate coefficient for the above reaction was found to be (6.2 0.6) x 109 l.mole-1.sec-1 at 23 °C. Scavenger studies indicated that G(CH3COCH3)T = 1.1. The anthracene negative ion was also observed. 

By contrast, in the pulse radiolysis of naphthalene and xenon mixtures in the gaseous phase, the triplet state of naphthalene was produced almost instantaneously ( 0.5 /isec)109. This was attributed to energy transfer by direct impact of sub-excitation electrons. 

The effect of solvent on the production of triplet states has been examined110. The yield of triplet states was relatively high in those non-polar solvents where they were not able to abstract hydrogen atoms. In solvents of intermediate polarity there is a decrease in the yield of triplet states accompanied by increased yields of 

Before closing this section describing photophysical phenomena, we illustrate them in the case of dyes attached to a polymer chains (Table 1.12).22) The ordinate shows the time range of the phenomena, which covers from 10 15 sec to 10° sec in usual solutions. 

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A specific unimolecular rate constant for the decay of a highly excited molecule at energy E and angular momentum J takes the fomr... 

Mullin A S and Schatz G C (eds) 1997 Highly Excited Molecules Relaxation, Reaction and Structure (ACS Symp. Ser. 678) (Washington, DC American Chemical Society)... 

Quack M and Tree J 1976 Unimolecular reactions and energy transfer of highly excited molecules Gas Kinetics and Energy Transfer mo 2, oh 5, ed P G Ashmore and R J Donovan (London The Chemical Society) pp 175-238 (a review of the literature published up to early 1976)... 

Hold U, Lenzer T, Luther K, Reihs K and Symonds A C 2000 Collisional energy transfer probabilities of highly excited molecules from kinetically controlled selective ionization (KCSI). I. The KCSI technique experimental approach for the determination of P(E, E) in the quasicontinuous energy ranged. Chem. Phys. 112 4076-89... 

Nesbitt D J and Field R W 1996 Vibrational energy flow in highly excited molecules role of intramolecular vibrational redistribution J. Rhys. Chem. 100 12 735-56... 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

Pibel C D, Sirota E, Brenner J and Dai H L 1998 Nanosecond time-resolved FTIR emission spectroscopy monitoring the energy distribution of highly vibrationally excited molecules during collisional deactivation J. Chem. Phys. 108 1297-300... 

Plenary 10. Hiro-o Hamaguchi, e-mail address lilrama ,chem.s.u-tokvo.ac.ip (time and polarization resolved multiplex 2D-CARS). Two-dimensional (tune and frequency) CARS using broadband dye source and streak camera timing. Studies dynamic behaviour of excited (pumped) electronic states. Follows energy flow within excited molecules. Polarization control of phase of signal (NR background suppression). 

Okamoto H, Nakabayashi T and Tasumi M 1997 Analysis of anti-Stokes RRS excitation profiles as a method for studying vibrationally excited molecules J. Phys. Chem. 101 3488-93... 

The second excitation mechanism, impact scattering, involves a short range interaction between the electron and the molecule (put simply, a collision) which scatters the electrons over a wide range of angles. The usefiil feature of impact scattering is that all vibrations may be excited and not only the dipole active ones. As in Raman spectroscopy, the electron may also take an amount of energy hv away from excited molecules and leave the surface with an energy equal to Eq + hv. 

It is well known that the electron-impact ionization mass spectrum contains both the parent and fragment ions. The observed fragmentation pattern can be usefiil in identifying the parent molecule. This ion fragmentation also occurs with mass spectrometric detection of reaction products and can cause problems with identification of the products. This problem can be exacerbated in the mass spectrometric detection of reaction products because diese internally excited molecules can have very different fragmentation patterns than themial molecules. The parent molecules associated with the various fragment ions can usually be sorted out by comparison of the angular distributions of the detected ions [8]. 

Weller A 1961 Fast reactions of excited molecules Progress in Reaction Kinetics (Oxford Pergamon) pp 187-214... 

Flynn G W, Michaels C A, Tapalian C, Lin Z, Sevy E and Muyskens M A 1997 Infrared laser snapshots vibrational, rotational and translational energy probes of high energy collision dynamics Highly Excited Molecules Relaxation, Reaction, and Structure ed A Mullin and G Schatz (Washington, DC ACS)... 

In tlie first stage, where at first we have two excitons S, excitation jumps from one of tlie excited molecules to... 

Here (p, cp2Q and (p2 are the waveflmctions of tire non-excited and excited molecules if tliere is no interaction between tliem. In tire case we consider tire molecules do interact and as a result the dimer exlribits properties different from Arose of tire monomers it comprises. In particular, tire energy level of tire excited state is different from tire monomer—it is split into two states ... 

A dye molecule has one or more absorption bands in the visible region of the electromagnetic spectrum (approximately 350-700 nm). After absorbing photons, the electronically excited molecules transfer to a more stable (triplet) state, which eventually emits photons (fluoresces) at a longer wavelength (composing three-level system.) The delay allows an inverted population to build up. Sometimes there are more than three levels. For example, the europium complex (Figure 18.15) has a four-level system. 

This leads to the possibiUty of state-selective chemistry (101). An excited molecule may undergo chemical reactions different from those if it were not excited. It maybe possible to drive chemical reactions selectively by excitation of reaction channels that are not normally available. Thus one long-term goal of laser chemistry has been to influence the course of chemical reactions so as to yield new products unattainable by conventional methods, or to change the relative yields of the products. 

In principle, one molecule of a chemiluminescent reactant can react to form one electronically excited molecule, which in turn can emit one photon of light. Thus one mole of reactant can generate Avogadro s number of photons defined as one einstein (ein). Light yields can therefore be defined in the same terms as chemical product yields, in units of einsteins of light emitted per mole of chemiluminescent reactant. This is the chemiluminescence quantum yield which can be as high as 1 ein/mol or 100%. 

At 70—140°C, peroxide is vaporised. Peroxide vapor has been reported to rapidly inactivate pathogenic bacteria, yeast, and bacterial spores in very low concentrations (133). Experiments using peroxide vapor for space decontamination of rooms and biologic safety cabinets hold promise (134). The use of peroxide vapor and a plasma generated by radio frequency energy releasing free radicals, ions, excited atoms, and excited molecules in a sterilising chamber has been patented (135). 

Another useful technique for measuring the rates of certain reactions involves measuring the quantum yield as a function of quencher concentration. A plot of the inverse of the quantum yield versus quencher concentration is then made Stern-Volmer plot). Because the quantum yield indicates the fraction of excited molecules that go on to product, it is a function of the rates of the processes that result in other fates for the excited molecule. These processes are described by the rate constants (quenching) and k (other nonproductive decay to ground state). 

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