Hexadiyne and Peroxy radicals are investigated by non linear, femtosecond, Four Wave Mixing (FWM) techniques. Both species are relevant in the context of first aromatic ring formation and ignition mechanisms, respectively. Recording spectrally dispersed fs-FWM signals unveils molecular features not accessible with the common pump probe technique.
maximum is reached at 6.8 eV (~ 180 nm), which
is in the vicinity of the dissociating B 2A” – state.
The IP of DTBOO is at 8.78 eV [
Photo excitation is expected to excite DTBOO to a non-bonding orbital configurations breaking the molecule symmetrically at the O-O bond, thus producing vibrationally „hot‟ TBO (ter-butoxy) radicals. The absorption of photons below ~ 230 nm wavelength, however, correlates to a direct dissociation channel yielding TBOO radicals (ter-butylperoxy) as products (Fig. 2).
Setting the excitation laser wavelength to 295 nm yielded similar signal characteristics in the resonant approach compared to a fully offresonant excitation. Astonishingly, the early time response of the resonant and the non-resonant transients obtained with TDBOO appears similar. Evaluation of the transient signals do not support a higher TBO production in the resonant case as expected. Thus, the anticipated resonance seems not to play a major role in the observed dissociation mechanism.
Hexadiyne
The origin of the soot production remains an
interesting process that is not yet fully explored. In
the case of aliphatic fuels (alkanes, acetylene,
ethylene) a first ring must be formed by a
sequence of elementary reactions. Ring formation is
the key process for the production of polyaromatic
hydrocarbons in flames [
Benzene formation is assumed to occur by
recombination („self-reaction‟) of C3H3 radicals [
With infrared and Raman spectroscopy, Hopf
and coworkers [
We registered dispersed offresonant fs- time resolved four wave mixing (fs-FWM) signals obtained in a gaseous 1.1 mixture of pentane and 1,5hexadiyne (total pour pressure is ~10 mbar). Fs-FWM vides information about molecular dynamics - e.g. isomerization steps - occurring in the ground tronic state of the molecule upon photoexcitation. The method involves three nonlinear electric field interactions with a molecular ensemble to create a resulting fourth wave (Fig. 3). Two laser pulses interact simultaneously with the molecular sample and coherently excite a ro-vibrational ensemble of states that is covered by the spectral width of the laser pulses. Rotations and low energy torsion or bending vibrations are thus possibly excited. The third, delayed probe-laser pulse interrogates the evolution of the molecular ensemble. In the present experiment all laser pulses have the same wavelength (~800 nm), a typical duration of ~100 fs and a spectral width of ~11 nm (fwhm).
The early time response ( = 150 fs, Fig.4 top) is mainly characterized by a time–zero coherence spike followed by ( = < - 150 fs) a rotational feature, which is caused by a fast dephasing of the initially excited rotations. From these signals it is possible to determine even in the absence of rotational recurrences the main rotational constants (A,B,C) of a molecule with a one- to two-digit accuracy. Hence, in some cases the early time response can be a sufficient indication to differentiate between molecules. However molecules, such as pentane and 1,5 hexadiyne have analogue rotational constants and therefore can not be readily distinguished by this method.
When the fs-FWM signal is spectrally dispersed additional information about the involved molecules is gained. Weak spectral features are observed in the dispersed signal (Fig 4 bottom) at delays < 1ps that are not discernible in the undispersed, one dimensional presentation (Fig 4 top). At a delay position of e.g. 0.7 ps, the spectral analysis of the signal fields enables the distinction between the mixed 1,5-hexadiyne sample and pure pentane (Fig. 5). The observed spectral peaks at this delay are separated by ~ 75 cm-1 and ~ 50 cm-1, respectively. Figure 6 (top) shows a “wavenumber” plot of the measurements shown in Figure 5. The abscissa shows the Fourier transformed signal along the delay time while the ordinate scales relatively to the carrier frequency of the signal pulse at ~ 1250 cm-1: beating frequencies are presented as a function of the signal spectrum.
Fig . 6: Top: ‘2D- wavenumber’ plot of the signal in Figure 2 bottom. Bottom: Raman spectrum of trans 1,5 hexadiyne.
We observe partial accordance between the calculated Raman spectrum (Fig. 6 bottom) of trans-1,5hexadiyne and the 2D-experiment. The low frequency mode appearing at ~ 320 cm-1 was expected from the experiment as the bandwidth of the laser is broad enough to cover this energy. However, the appearance of higher frequency components that are not, or only weakly, covered by the bandwidth of the laser pulses still needs more considerations.
The signal field in off-resonant degenerated FWM typically reflects the spectral profile of the input beams. Therefore, in the absence of isomerization processes we expect signal contributions replicating the spectral distribution of the input. However, if dynamics are effective between the input field interactions, as indicated in Figure 3. spectral departures form the input profile may become apparent, and may be interpreted adequately. Currently, detailed analysis of the measured data is in progress. 13. Huntsman, W.D. and H.J. Wristers, Thermal Rearrangement Of 1,5-Hexadiyne And Related Compounds. Journal Of The American Chemical Society, 1967. 89(2): p. 342-&.