=Paper=
{{Paper
|id=Vol-452/paper-6
|storemode=property
|title=Studies of Ter-Butyl-Peroxy and Hexadiyne by dispersed fs-FWM methods
|pdfUrl=https://ceur-ws.org/Vol-452/paper6.pdf
|volume=Vol-452
}}
==Studies of Ter-Butyl-Peroxy and Hexadiyne by dispersed fs-FWM methods==
https://ceur-ws.org/Vol-452/paper6.pdf
Studies of Ter-Butyl-Peroxy and Hexadiyne by dispersed fs-FWM methods
G. Knopp, P. Radi, A. Bodi, M. Johnson, T. Gerber*
Paul Scherrer Institut, Molecular Dynamics,
5232 Villigen/PSI, CH
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. Re-
cording spectrally dispersed fs-FWM signals unveils molecular features not accessible with the common pump probe
technique.
Introduction maximum is reached at 6.8 eV (~ 180 nm), which
2
Unimolecular dissociation is the extension of a is in the vicinity of the dissociating B A” – state.
chemical bond to infinity and occurs typically on The IP of DTBOO is at 8.78 eV [4] which is also
the sub picosecond timescale of molecular vibra- clearly seen by the appearance of a second pla-
tions. There exists a large variety of spectroscopic teau in the absorption spectrum of Fig.1. The
techniques that employ ultrashort laser pulses to weak-modulation of the absorbance just below the
monitor fast dynamic processes in molecules [1-3]. Ionization threshold energies may indicate the
presence of several Rydberg states.
Di-Tert-Butyl-Peroxide Photo excitation is expected to excite DTBOO
Di-tert-butyl peroxide (DTBOO) a sufficiently to a non-bonding orbital configurations breaking
stable molecule is a good candidate as precursor the molecule symmetrically at the O-O bond, thus
for the production and investigation of C4 – OO producing vibrationally „hot‟ TBO (ter-butoxy) radi-
peroxy radicals. The most prominent absorption cals. The absorption of photons below ~ 230 nm
( *-- *) in DTBOO is in the UV spectral range be- wavelength, however, correlates to a direct disso-
tween 340 nm and 230 nm. In order to learn more
about the state manifolds connecting to the states
of the fragments we started single photon, vacuum
ultraviolet, measurement at the SLS/VUV beam-
line. The VUV absorption of DTBOO has been
measured in the currently installed PEPICO end-
station. Hydrocarbon compounds can be intro-
duced into the endstation vessel at pressure up to
40 mbar without any risk of contamination of the
-9
beamline optics maintained in vacuum at 10
mbar. The absorbance for photon energies ranging
from 5.43 to 9.76 eV is shown in Figure 1 with a
spectral resolution of < 10 meV. The DTBOO va-
pour pressure in the chamber was 0.1 mbar and
the absorption length was ~ 30 cm. The absorption
cross section increases above 5.63 eV (below Fig.2: DTBOO fs-FWM excitation scheme
220 nm) by orders of magnitude and a relative
ciation channel yielding TBOO radicals (ter-butyl-
peroxy) as products (Fig. 2).
Setting the excitation laser wavelength to
295 nm yielded similar signal characteristics in the
resonant approach compared to a fully off-
resonant 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 dissocia-
tion mechanism.
Fig. 1: VUV absorption signal from 5.43 to 9.76 eV.
* Corresponding author: thomas.gerber@psi.ch
Towards Clean Diesel Engines, TCDE 2009
Hexadiyne vides information about molecular dynamics - e.g.
The origin of the soot production remains an in- isomerization steps - occurring in the ground
teresting process that is not yet fully explored. In tronic state of the molecule upon photoexcitation.
the case of aliphatic fuels (alkanes, acetylene, The method involves three nonlinear electric field
ethylene) a first ring must be formed by a se- interactions with a molecular ensemble to create a
quence of elementary reactions. Ring formation is resulting fourth wave (Fig. 3). Two laser pulses
the key process for the production of polyaromatic interact simultaneously with the molecular sample
hydrocarbons in flames [5]. It is believed that chain and coherently excite a ro-vibrational ensemble of
lengthening of acetylene leads to the formation of states that is covered by the spectral width of the
unsaturated radicals, which might stabilize by ring- laser pulses. Rotations and low energy torsion or
closure [6]. Based on RRKM calculations it was
concluded that chemically activated intermediates
can form aromatic rings faster than bimolecular
collisions, and therefore fundamental dissociation,
isomerisation or H atom loss reactions gain impor-
tance [7].
Benzene formation is assumed to occur by re-
combination („self-reaction‟) of C3H3 radicals [8-10]
via the “loose” transition states of 1,5-hexadiyne,
1,2-hexadiene-5-yne and 1,2,4,5-hexatetraene.
Calculations of the potential energy surfaces
(PES), temperature dependent branching ratios
and rate coefficients of the propargyl recombina-
tion reaction were accomplished by Miller and
Klippenstein [11]. There is a general agreement
that these intermediates are the primary recombi-
nation products that interconvert and isomerize to
secondary products eventually leading to ben-
zene[12]. The thermal rearrangement of 1,5-
hexadiyne, e.g., has been investigated between
210° to 350° C. Isomerization has been observed
over the whole temperature range towards a single
product, which has been identified as 3,4-
dimethylenecyclobutene [13].
With infrared and Raman spectroscopy, Hopf
and coworkers [8, 9] investigated the conformation Fig. 4: Top: Frequency integrated fs-FWM signal from
and vibrational spectra of 1,5-hexadiyne (bi- a 1,5 hexadiyne/pentane mixture. Bottom: Dispersed
fs-FWM signal. The delay between pump and probe
propargyl), 1,2-hexadiene-5-yne (propargylallene)
evolves to the left.
and 1,2,4,5-hexatetraene (biallenyl), in detail. It is
known that the dominant conformer in the vapour bending vibrations are thus possibly excited. The
phase of all three species is the trans- third, delayed probe-laser pulse interrogates the
configuration. Usually a trans-conformation is rec- evolution of the molecular ensemble. In the pre-
ognizable due to the mutual exclusion of infrared sent experiment all laser pulses have the same
and Raman bands. However, this is not true for the wavelength (~800 nm), a typical duration of ~100
conformers of benzene. Both band types show fs and a spectral width of ~11 nm (fwhm).
characteristic lines of The early time response ( = 150 fs, Fig.4 top)
trans- and gauche- is mainly characterized by a time–zero coherence
conformers, e.g., of spike followed by ( = < - 150 fs) a rotational fea-
bipropargyl. ture, which is caused by a fast dephasing of the
We registered initially excited rotations. From these signals it is
dispersed off- possible to determine even in the absence of rota-
resonant fs- time tional recurrences the main rotational constants
resolved four wave (A,B,C) of a molecule with a one- to two-digit accu-
mixing (fs-FWM) racy. Hence, in some cases the early time re-
signals obtained in a sponse can be a sufficient indication to differenti-
gaseous 1.1 mixture ate between molecules. However molecules, such
Fig.3: Fs-FWM excitation of pentane and 1,5- as pentane and 1,5 hexadiyne have analogue rota-
scheme. All transitions may hexadiyne (total
occur on one conformer or tional constants and therefore can not be readily
pour pressure is ~10 distinguished by this method.
partitioned, as indicated, on mbar). Fs-FWM
two distinct conformers.
When the fs-FWM signal is spectrally dispersed We observe partial accordance between the calcu-
additional information about the involved mole- lated Raman spectrum (Fig. 6 bottom) of trans-1,5-
cules is gained. Weak spectral features are ob- hexadiyne and the 2D-experiment. The low fre-
-1
served in the dispersed signal (Fig 4 bottom) at quency mode appearing at ~ 320 cm was ex-
delays < 1ps that are not discernible in the undis- pected from the experiment as the bandwidth of
persed, one dimensional presentation (Fig 4 top). the laser is broad enough to cover this energy.
At a delay position of e.g. 0.7 ps, the spectral However, the appearance of higher frequency
analysis of the signal fields enables the distinction components that are not, or only weakly, covered
between the mixed 1,5-hexadiyne sample and 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 replicat-
ing the spectral distribution of the input. However,
if dynamics are effective between the input field
interactions, as indicated in Figure 3. spectral de-
partures form the input profile may become appar-
ent, and may be interpreted adequately. Currently,
detailed analysis of the measured data is in
progress.
Fig. 5: Fs-FWM signal spectrum from pure pentane
(100%) and from the 1,5-hexadiyne/pentane mixture
(50%) at 0.7 ps probe delay. References
1. Mukamel, S., Principles of nonlinear optical spec-
pure pentane (Fig. 5). The observed spectral troscopy. 1995, New York [etc.]: Oxford University
-1 Press. XVIII, 543 S.
peaks at this delay are separated by ~ 75 cm and
-1
~ 50 cm , respectively. Figure 6 (top) shows a 2. Shen, y.R., The principles of nonlinear optics. 1984,
New York a.o.: Wiley. XII, 563.
“wavenumber” plot of the measurements shown in
3. Boyd, R.W., Nonlinear optics. 2nd ed. 2003, Amster-
Figure 5. The abscissa shows the Fourier trans- dam: Academic Press. 578 S.
formed signal along the delay time while the ordi- 4. NIST: http://webbook.nist.gov/chemistry/
nate scales relatively to the carrier frequency of the 5. Richter, H. and J.B. Howard, Formation of polycyclic
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signal pulse at ~ 1250 cm : beating frequencies aromatic hydrocarbons and their growth to soot - a
are presented as a function of the signal spectrum. review of chemical reaction pathways. Progress in
Energy and Combustion Science, 2000. 26(4-6): p.
565-608.
6. Crittenden, B.D. and R. Long, Formation of polycyclic
aromatics in rich premixed acetylene and ethylene
flames. Combustion and Flame, 1973. 20(3): p. 359-
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7. Westmoreland, P.R., A.M. Dean, J.B. Howard, and
J.P. Longwell, Forming Benzene In Flames By
Chemically Activated Isomerization. Journal Of Phys-
ical Chemistry, 1989. 93(25): p. 8171-8180.
8. Hopf, H., Base Catalysed Isomerisations of Bipro-
pargyl, Propargylallene, Biallenyl and Other Acyclic
C6h6-Isomers. Chemische Berichte-Recueil, 1971.
104(10): p. 3087-&.
9. Hopf, H., Thermal Isomerisations .3. Acyclic C6h6
Isomers. Chemische Berichte-Recueil, 1971. 104(5):
p. 1499-&.
10. Miller, J.A. and C.F. Melius, Kinetic And Thermody-
namic Issues In The Formation Of Aromatic-
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Figure 2 bottom. Bottom: Raman spectrum of trans -
1,5 hexadiyne.
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