Dye Laser Photolysis of n-Pentanal from 280 to 330 nm

Abstract

The UV photolysis of n-pentanal in the 280-330-nm region has been studied in 5-nm intervals by using dye laser photolysis in combination with cavity ring-down spectroscopy. Absorption cross sections of n-pentanal were measured at each wavelength studied. n-Pentanal exhibited a broad, structureless absorption band similar, in appearance, to that of previously studied short-chain aldehydes. The absorption spectrum peaked at 295 nm with a cross section of (6.56 ( 0.17) × 10 -20 cm 2 molecule -1 . The formation of the HCO radical, which is a photodissociation product, was monitored in these experiments. The HCO yield was found to be independent of n-pentanal pressure (2-18 Torr) and total pressure (8-480 Torr) except for 325-and 330-nm photolysis where the size of the HCO signal was small and the dissociation was near the threshold. The dependence of the HCO radical yield on the photolysis wavelength was determined. The HCO yields were 0.058 ( 0.006, 0.095 ( 0.009, 0.10 ( 0.02, 0.14 ( 0.01, 0.10 ( 0.02, 0.15 ( 0.02, 0.14 ( 0.02, 0.20 ( 0.06, 0.14 ( 0. 02, 0.085 ( 0.034, 0.087 ( 0.015 at 280, 285, 290, 295, 300, 305, 310, 315, 320, 325, and 330 nm, respectively, where uncertainty reflects experimental scatter only. End products from closed-cell photolysis of n-pentanal with and without O 2 were investigated at 290, 310, and 330 nm by using GC/MS and HPLC. Acetaldehyde was found to be a significant product from the photodissociation of n-pentanal/N 2 mixtures. Photolysis rates of n-pentanal to form HCO were calculated for two representative atmospheric conditions (noontime at sea level and 40°N latitude on January 1 and on July 1). The estimated radical formation rate constants from n-pentanal photolysis were about twice as fast as those obtained from acetaldehyde photolysis.

Photooxidation of n-Hexanal in Air

“…Photodissociation of aldehydes produces free radicals, which are subsequently involved in other atmospheric reactions and reaction cycles. There have been a number of studies devoted to the photodissociation of the simplest alkyl aldehydes, such as HCHO, CH 3 CHO, C 2 H 5 CHO [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ], but a relatively small number devoted to longer chain aldehydes, such as C 3 H 7 CHO, C 4 H 9 CHO [ 17 , 18 ] and iso -pentanal and tert -pentanal [ 32 ].…”

Cavity ring-down spectroscopic study of acetaldehyde photolysis in the gas phase, on aluminum surfaces, and on ice films

“…The stainless-steel reaction cell for gas-phase study has been described in detail elsewhere [9,11,16], and is schematized in Fig. 1, to allow comparison with the cell used for the surface studies.…”

“…Due to the crucial role played by aldehyde photolysis in the formation of HO x radicals in the atmosphere, the gas-phase photolysis of formaldehyde and acetaldehyde have been studied extensively [3][4][5][6]. Our group has previously studied the wavelength-dependent photolysis of propionaldehyde [7], n-butyraldehyde [8], i-butyraldehyde [8], n-pentanal [9], i-pentanal [10], t-pentanal [10], n-hexanal [11], and n-heptanal [11] in the gas phase, by using laser photolysis combined with cavity ring-down spectroscopy [12,13]. However, little is known about the product channels and quantum yields associated with aldehyde photolysis on the surface of snowpack.…”

“…Although pathway (2g) is an exothermic channel, there is a barrier for formation of the CH 4 + CO products. Our previous studies conducted on the analogous C3-C5 aldehyde systems showed that the molecular elimination channel becomes important at photolysis wavelengths shorter than 290 nm [7][8][9][10][11]. Pathway (3g) is another radical-formation channel, and is found to be minor for aliphatic aldehydes [15].…”

Photochemistry of aldehyde clusters: cross-molecular versus unimolecular reaction dynamics

“…There is evidence that process B can occur either in the ground state or in the triplet state. 16 Previous studies on the photochemistry of aliphatic aldehydes at atmospherically relevant photolysis wavelengths focused on gasphase photodissociation [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] and its pressure dependence. [31][32][33][34][35] Photolysis of aldehydes in a condensed matter environment, found for example in atmospheric aerosols, cloud and fog droplets, and in thin films formed on urban surfaces has not been systematically investigated.…”