CO vibrational energies

Figure 2 shows the results of two typical experiments. The observed CO vibrational energy distribution produced from the reactions 0(3P) + C2H2 and... 

Generally, good agreement was found between the experimentally observed and the statistically predicted population distributions assuming diradicals (as shown above) are formed initially instead of their alkene isomers. Table I summarizes results which show the comparison between the experimentally determined average product CO vibrational energies and the statistically expected values. 

Table I. Average CO Vibrational Energies and Absolute Rate Constants for CO Production at 300K for 0(3P) + 1-Alkynes.

Exothermicities and average CO vibrational energies (in units of kcal/mole) were computed by assuming that the reactions involve triplet CH2, except with C02 which is reactive only with excited singlet CH2 (see the text). 

The results of simple statistical calculations based on the RRK and RRKM theories of unimolecular reactions agree well with the observed vibrational population distribution. At 293 K, the computed average CO vibrational energy is 9.8 kcal/mole according to RRK and 10.2 according to RRKM calculations. Both agree quantitatively with the experimental value, 9.9 0.5. 

Average CO vibrational energy in kcal/mole. Statistically expected values. 

Fig. 21. Relative CO vibrational energy distributions from the 0( R) + CS and CSj reactions. Open circles distribution obtained from the flash photolysis of SOj, CS2, N2O, and Ar mixtures above 200 nm, filled circles distribution obtained from the flash photolysis of NOj, CS2, and SFj mixtures above 300 nm. Both are normalized to c=I. (Ref. 145.) Triangles Data of Hancock et al. obtained from the O + CS reaction, (Ref. 147) normalized to c= 13 of the open-circle distribution. The distributions below c 5 are attributable to the O+CSj CO+ 2 reaction.

Some detailed calculations have been made by Tully [209] on the trajectories for Rideal-type processes. Thus the collision of an oxygen atom with a carbon atom bound to Pt results in a CO that departs with essentially all of the reaction energy as vibrational energy (see Ref. 210 for a later discussion). 

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

Midey A J and Viggiano A A 1998 Rate constants for the reaction of Ar" with O2 and CO as a function of temperature from 300 to 1400 K derivation of rotational and vibrational energy effects J. Chem. Phys. at press... 

L of CO was adsorbed at a pressure of 1 x 10 mbar and T= 200 K. At zero energy loss one observes the highly intense elastic peak. The other peaks in the spectrum are loss peaks. At high energy, loss peaks due to dipole scattering are visible. In this case they are caused by CO vibration perpendicular to the surface. The... 

IS] Brueok S R J and Osgood R M Jr 1976 Vibrational energy relaxation in liquid N2-CO mixtures Chem. Phys. Lett. 39 568-72... 

Figure 1 Schematic of eiectron energy-loss scattering process for electrons of energy striking a Rh single-crystal surface with adsorbed CO molecules present. The actual energy-loss spectrum, due to excitation of CO vibrations, is shown also.

Figure 10.6 Graph of the Boltzmann distribution function for the CO molecule in the ground electronic state for (a), the vibrational energy levels and (b), the rotational energy levels. Harmonic oscillator and rigid rotator approximations have been used in the calculations.

The vibrational relaxation of simple molecular ions M+ in the M+-M collision (where M = 02, N2, and CO) is studied using the method of distorted waves with the interaction potential constructed from the inverse power and the polarization energy. For M-M collisions the calculated values of the collision number required to de-excite a quantum of vibrational energy are consistently smaller than the observed data by a factor of 5 over a wide temperature range. For M+-M collisions, the vibrational relaxation times of M+ (r+) are estimated from 300° to 3000°K. In both N2 and CO, t + s are smaller than ts by 1-2 orders of magnitude whereas in O r + is smaller than t less than 1 order of magnitude except at low temperatures. 

A simple phase space model can be used to compute the CO product vibrational energy distribution as a function of the available energy,12-14 Eav. The maximum energy which can be partitioned among the products degrees of freedom is the reaction exoergicity, Ex = hv-DH°[(CO)5W-CO]. For a 351 nm photolysis,... 

Ex 35 Kcal/mole.15 We find that the CO product vibrational distribution calculated using the phase space model with Eav = 35-40 Kcal/mole is in good agreement with our experimental results (Figure 2). Thus, the measured CO vibrational distribution indicates that vibrational energy disposal to the photolysis products is determined at a point on the potential surface where the full reaction exoergicity is available. This suggests that the 351 nm excitation of W(CO)g results in the sequence of events, (2)-(4), where the asterisk denotes vibrational excitation. 

Figure 2. Vibrational energy distribution of the CO product formed via the 351 nm photolysis of W(CO)s. Experimental data are indicated asfl. The lines correspond to results obtained by phase space calculations with an available energy of 40 and 35 Kcal/mole.

Conclusions. Time-resolved CO laser absorption spectroscopy can provide information useful in characterizing the primary photochemical channels in gas-phase transition metal carbonyls. We have found that product vibrational energy distributions indicate that W(CO)g and Cr(CO>6 dissociate via different... 

We have also investigated the application of Franck-Condon models in fitting the CO product vibrational energy distribution and find they are unable to do so. D. Anderson, J. Brown, R. Fletcher, K. Prather and R. Rosenfeld, manuscript in preparation. 

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