Mass-Resolved Momentum Imaging of Three Dichloroethylene Isomers by Femtosecond Laser-Induced Coulomb Explosion
Round 1
Reviewer 1 Report
The paper by Wada et al. presents mass resolved momentum imaging of three dichloroethylene (DCE) isomers by using a laser Coulomb explosion technique. Although the basic idea for distinguishing these isomers was demonstrated by other groups(ref.24), the authors showed that all three isomers can be more clearly separated according to their momentum distributions, which are sensitive to laser intensities and the relative configuration with respect to the laser polarization. The results are well presented and would be of interest in the professional community. Overall, the paper deserves to be published in Photochem. However, I have a few minor comments and questions that the authors should consider.
1) In Fig.2, it would be useful for readers to include expanded figures of C2+ in addition to those of 35Cl+ as the C2+ ion is the key to the current work.
2) Some images in Figs. 5&6 show a slight tilt with respect to the direction of the laser polarization axis. Can the authors make some comment on that? Could the tilt be related to a dynamical alignment or structural deformation during the interaction with intense lasers? If so, how large is the influence against the present isomer separation method?
3) The authors discuss multiple ionization of DCE isomers in terms of the most polarizable axis presented in Fig.7. As the weakest laser intensity of 1.8×1014 W/cm2 in the present study can be still strong enough for DCEs to undergo tunnel ionization which is more sensitive to a shape of a molecular orbital, it is not clear to me how the authors want to interpret the experimental data by the most polarizable axis in Fig.7.
4) Have the authors checked momentum distributions of C2Hn+ (n=0-2) ions? The ions from three DCE isomers could have specific momenta by the recoil against the counterpart Cl atoms when a prompt dissociation occurs.
Author Response
Response to the comments by Reviewer 1
1) COMMENT: In Fig. 2, it would be useful for readers to include expanded figures of C in addition to those of 35Cl+as the C2+ ion is the key to the current work.
REPLY: Following the reviewer’s comment, we have added expanded spectra of C2+ ion as the insets in Figs. 2a-c. Additionally, we have revised the figure caption, and some sentences in the second paragraph on page 4.
2) COMMENT: Some images in Figs. 5&6 show a slight tilt with respect to the direction of the laser polarization axis. Can the authors make some comment on that? Could the tilt be related to a dynamical alignment or structural deformation during the interaction with intense lasers? If so, how large is the influence against the present isomer separation method?
REPLY: A slight tilt shown in some images is caused by fluctuation in the measurements. Although dynamical alignment and structural deformation can be induced by a fs laser field, these effects do not break symmetry with respect to the laser polarization. The image should be symmetric along both the parallel and perpendicular directions with respect to the laser polarization direction. The momentum distribution shown in the first quadrant must be the same as the three other quadrants. In the measurement of time-of-flight spectra, the polarization angles were set to be every 10 deg. in the ranges of 0 - 90 deg. and of 95 - 175 deg. The angle range of 95 - 175 is equivalent to the range of 5 - 85 deg. In the analysis done in the previous manuscript, we did not impose the symmetry requirement and made MRMI maps in the range of polarization angle, theta = 0 - 180 deg. The symmetry requirement was taken into account for angular distributions in Figs. 3 and 4. However, we did not do so for MRMI maps mistakenly. We thank the reviewer for letting us notice this point.
If we could obtain the data with small statistical fluctuation, even the previous treatment should show symmetric images. However, in the present experiment, the statistical fluctuation was quite large and caused non-symmetric images. In the revised manuscript, we treat the polarization angle theta in the range of 95 - 175 deg. as (180 - theta) deg. Consequently, the MRMI maps are obtained from the TOF data with interval of 5 deg. in the range of theta = 0 - 90 deg. Density of data point for making MRMI contour maps becomes twice, resulting in better statistics.
Despite the fairly large fluctuation in the measurement, all three isomers in DCE can be identified by the present data, indicating the robustness of the present approach.
At the end of Subsection 2.2, we have added a description on the procedure of our analysis.
3) COMMENT: The authors discuss multiple ionization of DCE isomers in terms of the most polarizable axis presented in Fig. 7. As the weakest laser intensity of 1.8x1014 W/cm2 in the present study can be still strong enough for DCEs to undergo tunnel ionization which is more sensitive to a shape of a molecular orbital, it is not clear to me how the authors want to interpret the experimental data by the most polarizable axis in Fig. 7.
REPLY: As pointed out by the reviewer, tunnel ionization is sensitive to a shape of MO. However, in the present study, we could not identify which orbitals are contributing in the ionization. Therefore, we simply assume that the molecules whose most polarizable axis is oriented along the laser polarization are preferentially ionized into multiply charged ions.
To explain this, we have revised some sentences in the first and second paragraph of Discussion (page 8).
4) COMMENT: Have the authors checked momentum distributions of C2Hn+ (n = 0 - 2) ions? The ions from three DCE isomers could have specific momenta by the recoil against the counterpart Cl atoms when a prompt dissociation occurs.
REPLY: As pointed out by the reviewer, momentum distributions of C2Hn+ ions are quite interesting. However, in the present experiments, we did not obtain any characteristic distributions for the ion signals (m/z 24 - m/z 28). We have made no revision to the present context.
Reviewer 2 Report
The reported results are interesting and the manuscript could be recommended to be considered for publication.
I could recommend the authors mention the cluster generation process inside the chamber, discuss its density contribution in comparison with the remained gas phase density, and then consider its effects on the output MCP signal. A similar approach could be cited in Applied Physics Letters 102, 191106 (2013).
Author Response
Response to the comments by Reviewer 2
COMMENT: I could recommend the authors mention the cluster generation process inside the chamber, discuss its density contribution in comparison with the remained gas phase density, and then consider its effects on the output MCP signal. A similar approach could be cited in Applied Physics Letters 102, 191106 (2013).
REPLY: In the present experiments, the backing pressure of the pulsed valve was kept below 1.3x103 Pa, to avoid negative effects on the momentum images due to clustering and space-charge effects. In fact, the dimer cations (m/z96 - 100) were not observed in the present experimental conditions. Thus, we consider that the influence of the cluster generation was negligible.
To mention the backing pressure, we have added two sentences and cited the suggested reference as [31] in the second paragraph of Experiment and Analysis (page 2).
