Photochemistry experiments

Cold, with well-defined internal energy. This is normally ensured with supersonic cooling. This requirement is important for high resolution and state-resolved studies, and it can remove the spectroscopy congestion of the radicals and enhance the interpretation of the results in photochemistry experiments. 

The hypothesis that the cobalt carbonyl radicals are the carriers of catalytic activity was disproved by a high pressure photochemistry experiment /32/, in which the Co(CO), radical was prepared under hydroformylation conditions by photolysis of dicobalt octacarbonyl in hydrocarbon solvents. The catalytic reaction was not enhanced by the irradiation, as would be expected if the radicals were the active catalyst. On the contrary, the Co(C0)4 radicals were found to inhibit the hydroformylation. They initiate the decomposition of the real active catalyst, HCo(C0)4, in a radical chain process /32, 33/. 

Cirkva and Hajek have proposed a simple application of a domestic microwave oven for microwave photochemistry experiments [86]. In this arrangement, the EDL (the MW-powered lamp for this application was specified as a microwave lamp or MWL) was placed in a reaction vessel located in the cavity of an oven. The MW field generated a UV discharge inside the lamp that resulted in simultaneous UV and MW irradiation of the sample. This arrangement provided the unique possibility of studying photochemical reactions under extreme thermal conditions (e.g. Ref. [87]). 

Fig. 14.5 A modified MW oven for microwave photochemistry experiments. A. magnetron, B. reaction mixture with the EDL and a magnetic stir bar, C. aluminum plate, D. magnetic stirrer, E. infrared pyrometer, F. circulating water in a glass tube, G. dummy load inside the oven cavity [88]. With permission from Elsevier Science.

Photochemical Routes. Finally, two sets of experiments from Rest s laboratory which demonstrate the subtle implications of quite complex matrix photochemistry experiments. The first (67) involves (n -C5H5)Co(CO)2 and can be summarised... 

Abstract. Model-measurement comparisons of HOx in extremely clean air ([NO]<3 ppt) are reported. Measurements were made during the second Southern Ocean Photochemistry Experiment (SOAPEX-2), held in austral summer 1999 at the Cape Grim Baseline Air Pollution Station in northwestern Tasmania, Australia. 

This paper investigates the radical chemistry of the clean marine boundary layer in the Southern Ocean during the SOAPEX-2 (Southern Ocean Photochemistry Experiment 2) campaign using an observationally constrained box-model based on the Master Chemical Mechanism (Jenkin et al., 1997, 2003 Saunders et al., 2003). The primary aim of SOAPEX-2 was to study free radical chemistry in the remote marine boundary layer in the Southern Hemisphere. Sections 2 and 3 of this paper describe the SOAPEX-2 site and the measurements that were made during the campaign. Section 4 describes the models used and Sect. 5 presents the results. Finally, Sect. 6 contains the summary and the conclusions. 

Shetter, R. E A. H. McDaniel, C. A. Cantrell, S. Madronich, and J. G. Calvert, Actinometer and Eppley Radiometer Measurements of the N02 Photolysis Rate Coefficient during the Mauna Loa Observatory Photochemistry Experiment, J. Geophys. Res., 97, 10349-10359 (1992). 

Talbot, R. W B. W. Mosher, B. G. Heikes, D. J. Jacob, J. W. Munger, B. C. Daube, W. C. Keene, J. R. Maben, and R. S. Artz, Carboxylic Acids in the Rural Continental Atmosphere over the Eastern United States during the Shenandoah Cloud and Photochemistry Experiment, / Geophys. Res., 100, 9335-9343 (1995). 

Greenberg, J. P D. Helmig, and P. R. Zimmerman, Seasonal Measurements of Nonmethane Hydrocarbons and Carbon Monoxide at the Mauna Loa Observatory during the Mauna Loa Observatory Photochemistry Experiment 2, J. Geophys. Res., 101, 14581-14598 (1996). 

Harder, J. W., R. O. Jakoubek, and G. H. Mount, Measurement of Tropospheric Trace Gases by Long-Path Differential Absorption Spectroscopy during the f993 OH Photochemistry Experiment, J. Geophys. Res., 102, 6215-6226 (1997a). 

Harder, J. W., A. Fried, S. Sewell, and B. Henry, Comparison of Tunable Diode Laser and Long-Path Ultraviolet-Visible Spectroscopic Measurements of Ambient Formaldehyde Concentrations during the 1993 OH Photochemistry Experiment, . /. Geophys. Res., 102, 6267-6282 (1997b). 

Mount G.H., F.L. Eisele, D.J. Tanner, J.W. Brault, P.V. Johnston, J.W. Harder, E.J. Williams, A. Fried, and R. Shetter. 1997. An intercomparison of spectroscopic laser long-path and ion-assisted in situ measurements of hydroxyl concentrations during the Tropospheric OH Photochemistry Experiment, fall 1993.. Geophys. Res. 102 6437-6455. 

Another approach to the production of UV photons includes the development of electrode-less discharge lamps driven by microwave excitation (e.g. Fassler et al., 2001, Ametepe et al., 1999, He et al., 1998). This type of lamp is shown in Fig. 4-17. In this case, the excitation of mercury vapor within the discharge gap is achieved by coupling in the energy with a water-cooled high-frequency spool. This concept may be a very convenient tool for microwave photochemistry experiments by simultaneous combination of microwave and VUV/UV irradiation of aqueous systems (c.f Klan et al., 2001, 1999). 

Eisele FL, Mount GH, Tanner D, Jefferson A, Shetter R, Harder JW, Williams EJ (1997) Understanding the Production and Interconversion of the Hydroxyl Radical During Tropospheric OH Photochemistry Experiment,/. Geophys. Res. 102, No. D5 6457-6465. 

Goldan P. D., Kuster W. C., and Fehsenfeld F. C. (1997) Nonmethane hydrocarbon measurements during the tropospheric OH photochemistry experiment. J. Geophys. Res. 

McKeen S. A., Mount G., Eisele F., Williams E., Harder J., Goldan P., Kuster W., Liu S. C., Baumann K., Tanner D., Fried A., Sewell S., Cantrell C., and Shetter R. (1997) Photochemical modeling of hydroxyl and its relationship to other species during the Tropospheric OH Photochemistry Experiment. J. Geophys. Res. 102, 6467-6493. 

Ridley B. A., Madronich S., Chatfield R. B., Walega J. G., Shetter R. E., Carroll M. A., and Montzka D. D. (1992) Measurements and model simulations of the photostationary state during the Mauna Loa Observatory Photochemistry Experiment implications for radical concentrations and ozone production and loss rates. J. Geophys. Res. 97, 10375-10388. 

Figure 1. (a) Schematic of the apparatus which may be employed in a surface photochemistry experiment. The Sample (S) is mounted on a temperature controlled block in UHV. It may be cleaned by argon ion bombardment. The adsorbate is dosed onto the surface (D). Its dark state is determined by TPD (as measured by mass spectrometry, MS), EELS, UPS etc. Irradiation is by an arc lamp via a monochromator or filters. Products arc detected by the same analysis tools. 

The most intense sources of UV radiation are the high-pressure ( 100 bar) mercury arcs. The spectral lines are broadened due to the high pressure and temperature and they are superimposed on a continuous background of radiation (Figure 3.4). While common mercury xenon [Hg(Xe)] lamps still produce significant mercury emission bands, especially in the UV region, the smoother xenon lamp spectrum finds application in environmental photochemistry experiments because of its resemblance to solar radiation (Figure 1.1). 

Cgo should be effectively quenched by the amino groups in the solution. Photochemistry experiments under better controlled conditions are required for a mechanistic understanding of these interesting and useful reactions. 

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