Title:
|
Chemical processes studied by time-resolved FTIR
|
This thesis has applied the time-resolved FTIR emission technique to the study of a number of
gas phase cheinical processes, including photodissociation, chemical reaction and energy
transfer. Two particular types of species are considered: vibrationally excited NO, formed from
photolysis, reaction and energy transfer pathways, and organic radical compounds formed from
photolysis and involved in secondary reaction processes.
Vibrationally excited NO has been observed as one of the reaction products following the
photolysis of N02 over a range of wavelengths in the region 193 - 327 nm. Emission from the
fundamental and first overtone bands was analysed to obtain the nascent vibrational
distribution at each wavelength considered. The nascent vibrational distribution was found to
be strongly inverted and wavelength dependent. Photolysis at A > 244 nm proceeds via one
reaction pathway;
1
A>244nm )NO+Oep) (1)
and the NO(v) distribution is of atmospheric importance in determining the global
concentrations of NO measured from satellite emission studies. There is a smooth trend in the
inverted distributions with increasing photon energy. Nascent populations were used to
produce a new model to predict the wavelength dependence of the nascent populations of
NO(v) formed following photolysis of N02 in the range 265 - 370 nm. Such work has been
shown to provide useful new information for atmospheric models of global concentrations of
NO.
Atmospheric models of NO concentrations also require input parameters for the rates of
relaxation of NO(v) in collision with quenching molecules, in particular 02. The rate constants
for vibrational quenching of NO(v = 1 - 16) in collisions with a number of species have been
measured. Vibrationally excited NO was produced by both the 193 nm photolysis of N02 and
the reaction of OeD) with N20, initiated by the 193 nm photolysis of N20. Quenching rates
were measured for collisions with O2, He, NO and CO2.
The vibrational distribution of NO(v) formed following photolysis of N02 at 193 nm was
confirmed to be bimodal with a secondary maximum at v = 14 allowing new rates of relaxation
of highly vibrationally excited NO to be determined due to the relatively large nascent
populations of these levels. At A< 244 nm at second reaction channel for N02 photolysis opens
up and in order to assess the contribution of this channel to the overall distribution at such
wavelengths a new nascent distribution was determined following photolysis at 226 nm. The
distribution was found to be bimodal and could qualitatively be described by the sum of the
Oep) and OeD) channels.
Vibrationally excited NO was also formed following energy transfer from NO A z:E+(v = 0),
which was achieved by 226 nm pumping of ground state NO. Nascent vibrational distributions
were determined for a variety of collision partners and NO was shown to be populated up to at
least v = 20. The distributions obtained varied with the collision partners: NO, CO, COz and
Ar. Distributions of ground state NO(v) were also acquired under conditions such that the
internal energy in the products was governed by the emission coefficients. Comparison of
distributions obtained by collision and fluorescence and those predicted from the emission
coefficients allowed a purely collisional NO(v) distribution to be estimated when NO and CO
were the quenching molecules. Emission was also observed from high vibrational modes of
CO and COz indicating a direct energy transfer from NO A z:E\v = 0). Argon was found to not
significantly affect the distribution at the pressures employed but afforded a dramatic increase
in intensity from NO(v) that was shown to be due to hole burning and subsequent collisional
hole filling.
The second part of this thesis describes the generation and reactions of organic radical species.
The photolysis of vinyl bromide and vinyl chloride at 193 urn was considered and nascent
vibrational populations of the HBr and HCI products have been measured and found to be
indicative of a three-centre formation mechanism. Secondary reaction products of the vinyl
radical (CZH3) produced from this photolysis process have also been considered and
mechanisms for their formation proposed. The presence of buffer gas has been shown to
generate new features which have been assigned to vibronic emission from the secondary
reaction product CzH.
Products of the reaction of Oz with the vinyl radical, formed from the 193 nm photolysis of
vinyl bromide and vinyl chloride, have been identified. In particular the existence two primary
reaction channels have been confirmed by the assignment of emission from CO2, HCO and
HzCO, and the mechanism of one secondary reaction channel has been proposed by analysing
the kinetics of emission from CO(v). In addition a new reaction channel has been proposed by
considering emission from CzHz. A qualitative branching ratio between these three reaction
channels has been proposed. The reaction of CZH3 with O2 is of significance to combustion
processes and a rate constant for the process has been determined to be 9.5 x 1O,tZ cm3
molecule,l S'I by assessing the kinetics of emission from HzCO.
As a comparison to the generation of simple unsaturated free radicals, such as the vinyl radical,
the photolysis of acetone at 266 nm to generate the simple saturated methyl radical was
investigated. Primary reaction products were identified as the CH3 and CH3CO radicals and
emission was observed in the C-H stretching region that was assigned on the basis of its
kinetics to the methyl radical recombination product CZH6• CO(v = 1 - 6) was also observed as
a secondary product of the process and attributed to photolysis of CH3CO on account of its
significantly hotter nascent vibrational distribution than that obtained in previous studies at 193nm.
|