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Imaging transient species in the femtosecond -band photodissociation of
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10.1063/1.3236808
/content/aip/journal/jcp/131/13/10.1063/1.3236808
http://aip.metastore.ingenta.com/content/aip/journal/jcp/131/13/10.1063/1.3236808
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Figures

Image of FIG. 1.
FIG. 1.

Schematic representation of the relevant potential energy surfaces along the C–I bond representing the -band photodissociation of . A nuclear wave packet initially prepared in the potential energy curve by a 267 nm femtosecond laser pulse evolves adiabatically on that surface yielding . A portion of the initial wave packet leaks nonadiabatically to the surface, which correlates with . The main aim of the present study is to follow the real time dynamics of the photodissociation process from reagents to products through transient species by probing with nonresonant MPI using an IR probe pulse (802 nm).

Image of FIG. 2.
FIG. 2.

Pump-probe transient ion signal of the parent (a) and (b) and their corresponding fragments , , and obtained by integrating each mass peak in the oscilloscope trace. The different transients in each plot are normalized for clarity of presentation. The lines correspond to fits to an exponential decay convoluted with a Gaussian representing the cross correlation (FWHM of ) between the pump (267 nm) and probe (802 nm) pulses. For , the decay times found for parent, , and are , , and , respectively. For , the decay times for parent, , and are , , and , respectively. Time shifts between the parent ion and the fragment ion transients are found to be and for and , respectively, and and for and , respectively. Time zero is defined as the pump-probe delay time corresponding to the instantaneous rise of the exponential decay function used to fit the corresponding parent ion transient. It must be emphasized that given the width of the cross correlation, asymmetric transients would only be obtained for decay times .

Image of FIG. 3.
FIG. 3.

Abel-inverted images obtained upon excitation of at 267 nm for a pump-probe delay time of 1 ps. (a) detection by nonresonant MPI at 802 nm (at least seven photons are needed). The contrast of this image has been increased by a factor of 4. (b) detection by REMPI at 333.5 nm [ branch of the transition]. (c) Asymptotic CM KEDs obtained at 802 (red) and 333.5 nm (blue) probe wavelengths together with the assignments made for the different product channels. The curves have been normalized to clarify the comparison.

Image of FIG. 4.
FIG. 4.

False-color Abel-inverted images obtained as a function of pump-probe delay time. Color scale is kept constant so as to highlight the changes in overall signal intensity. The image corresponding to the asymptotic case (delay time of ) is intensified by a factor of 4 with respect to the others. The most intense ring corresponds to the main dissociation channel. A larger radius ring, which produces a much weaker signal, can be attributed to the channel. An additional channel of lower kinetic energy (smaller radius) and strong anisotropy can be seen when pump and probe pulses overlap. The weak but clear rings that can be observed in the image taken at a delay of corresponding to the asymptotic and -band dissociation channels keep a similar intensity for much longer delay times .

Image of FIG. 5.
FIG. 5.

Transients obtained by angular integration of the different rings observed in the images. (a) transient. The signal was extracted from the images as indicated in the text. (b) Ring assigned to the channel. (c) Ring assigned to the channel. (d) Inner ring. The transient sets the time zero in the pump-probe experiment. Note that the transients shown in (b) and (c) have the same delay time at maximum intensity of with respect to time zero. The transient shown in (d) has no delay time at maximum. Open circles: experimental data. Solid lines: fits to a molecular response function convoluted with a Gaussian representing the cross correlation (FWHM of ) between the pump (267 nm) and probe (802 nm) pulses. For the and inner ring transients, the molecular function is an exponential decay with a decay time of . For (b) and (c) transients the molecular function is made of an exponential decay added to a Boltzmann sigmoidal. The parameters of the fits are for the decay time and and for the sigmoidal curve. The ratios between the amplitudes of the exponential decay to the sigmoidal are 49 and 19 for transients (b) and (c), respectively.

Image of FIG. 6.
FIG. 6.

CM KEDs at selected pump-probe delay times as indicated in the insets, where the fitted curve corresponding to the transient shown in Fig. 5(b) is depicted along with color circles to clarify what delay times are represented: (a) from −400 to 110 fs and (b) from 110 to 520 fs. Peaks (1) and (2) correspond to the and channels, respectively. Peak (3) may correspond to a dissociative ionization channel. The labels on top of peak (1) indicate the values of the kinetic energy at the maximum of the peak.

Image of FIG. 7.
FIG. 7.

Representation of the CM kinetic energy at the maximum of peak (1) as a function of the pump-probe delay time. The error bars on each point represent the standard deviation of a number of sequences of images.

Image of FIG. 8.
FIG. 8.

(black dots) and (red dots) anisotropy parameters as a function of the pump-probe delay time obtained by radial integration of the images for the (a) and (b) channels. Error bars on each point correspond to the standard deviation of a number of sequences of images. For clarity the fitted transients shown in Fig. 5(b) and 5(c) are depicted in each panel. Vertical dashed lines indicate the limit from where the angular distributions can be fitted by using only (asymptotic case). See the text for more details.

Image of FIG. 9.
FIG. 9.

Alignment dynamics of represented by . This quantity is derived from the intense ring in the raw images assigned to the channel measured as a function of the delay time between an IR alignment pulse (802 nm) and the excitation pulse (267 nm). The fragment arising from the -band photodissociation of is probed at a long delay time (several picoseconds) by REMPI at 333.5 nm [ branch of the transition]. The first half revival is observed at a delay time between the alignment and photolysis pulses of about 32 ps, which corresponds to half of the rotational period of the molecule.

Image of FIG. 10.
FIG. 10.

Asymptotic (long delay time of 1 ps) CM KEDs obtained at 802 (red) and 333.5 nm (blue) probe wavelengths from the 267 nm photodissociation of together with the assignments made for the different product channels. The curves have been normalized for clarity of the comparison.

Image of FIG. 11.
FIG. 11.

(a) Same as in Fig. 6 but for . (b) Same as in Fig. 7 but for .

Image of FIG. 12.
FIG. 12.

Schematic representation of the energy levels of neutral and ionic parent molecule and fragments. Energy levels for the ionic channels taken from Ref. 25. The arrows indicate multiple 267 and 802 nm photons absorbed to reach the ionic ladder of .

Image of FIG. 13.
FIG. 13.

Schematic representation of the proposed mechanism for the detection of transient species and final products in the real time photodissociation of in the -band by multiple absorption of pump and probe laser photons. See the text for details.

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/content/aip/journal/jcp/131/13/10.1063/1.3236808
2009-10-07
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Imaging transient species in the femtosecond A-band photodissociation of CH3I
http://aip.metastore.ingenta.com/content/aip/journal/jcp/131/13/10.1063/1.3236808
10.1063/1.3236808
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