^{1,a)}, Atsushi Yokoyama

^{1}, Kaoru Yamanouchi

^{1,2}and Ryuji Itakura

^{1,b)}

### Abstract

The two dissociative ionization channels of ethanol (C2H5OH) induced by an intense near-infrared laser pulse (λ ∼ 783 nm), C2H5OH → CH2OH+ + CH3 + e− and C2H5OH → C2H5 + + OH + e−, are investigated using photoelectron-photoion coincidence method. It is shown that both the electronic ground state and the first electronically excited state of C2H5OH+ are produced at the moment of photoelectron emission. From the observed correlation between the electronic states of C2H5OH+ prepared at the moment of photoelectron emission and the kinetic energy release of the fragment ions, it is revealed that C2H5OH+ prepared in the electronic ground state at the photoelectron emission gains larger internal energy in the end than that prepared in the electronically excited state. The averaged internal energy of C2H5OH+ just before the dissociation is found to increase when the laser field intensity increases from 9 to 23 TW/cm2 and when the laser pulse duration increases from 35 to 800 fs.

The authors acknowledge support by Dr. K. Yokoyama, Dr. A. Sugita and Dr. Y. Fukuda in the construction of the experimental apparatus. This study was supported in part by the JAEA special research project of “Reaction control in intense laser fields”, by MEXT/JSPS KAKENHI (Grant Nos. 14077205, 17750004, 19685003, and 23350013), and by the Matsuo Foundation.

I. INTRODUCTION

II. EXPERIMENT

III. RESULTS AND DISCUSSION

A. Momentum images of product ions and photoelectrons

B. Channel-specific photoelectron spectra

C. Correlation map of kinetic energies of a photoelectron and a fragment ion

D. Translational temperature of the fragment ions

1. Dependence on the laser peak intensity

2. Dependence on the laser pulse duration

E. Relative yields of the product ions

IV. SUMMARY

### Key Topics

- Photoelectron spectra
- 26.0
- Ground states
- 24.0
- Ionization
- 24.0
- Excited states
- 21.0
- Excitation energies
- 20.0

## Figures

Schematic energy level diagram of C2H5OH+ measured from the ground state of neutral C2H5OH. A thick and open arrow in the upward direction denotes one photon absorption and a thin arrow in the downward direction denotes photoelectron emission. In the early stage of the laser pulse, multiphoton absorption and photoelectron emission proceed, and further photoabsorption can proceed within the same laser pulse to produce electronically excited C2H5OH+. The threshold energies for yielding the respective product ions are shown with short horizontal bars on the right side of the figure.

Schematic energy level diagram of C2H5OH+ measured from the ground state of neutral C2H5OH. A thick and open arrow in the upward direction denotes one photon absorption and a thin arrow in the downward direction denotes photoelectron emission. In the early stage of the laser pulse, multiphoton absorption and photoelectron emission proceed, and further photoabsorption can proceed within the same laser pulse to produce electronically excited C2H5OH+. The threshold energies for yielding the respective product ions are shown with short horizontal bars on the right side of the figure.

Experimental setup for the photoelectron photoion coincidence momentum imaging. The counter-plot of the electric field applied by the electrostatic lenses is drawn. In the top right, the directions of the coordinates (x, y, and z) are shown.

Experimental setup for the photoelectron photoion coincidence momentum imaging. The counter-plot of the electric field applied by the electrostatic lenses is drawn. In the top right, the directions of the coordinates (x, y, and z) are shown.

(a)–(d) 2D cuts of 3D momentum distributions of product ions with (a) C2H5OH+, (b) C2H4OH+, (c) CH2OH+, and (d) C2H5 +. (e)–(h) 2D sliced images of the 3D momentum distribution of photoelectrons detected in coincidence with (e) C2H5OH+, (f) C2H4OH+, (g) CH2OH+, and (h) C2H5 +. The laser peak intensity and pulse duration are I 0 = 9 TW/cm2 and τ = 35 fs. The vertical (y) axis is along the laser polarization direction. The unit of the coordinates is the atomic unit (a.u.) in both electron and ion images. On the top axes in (a)–(d), the velocity unit (km/s) is shown as a reference. An atomic unit of the momentum is equal to 1.198 u km/s. In (a)–(d), p Ixz is defined as |p Ixz | = (p Ix 2 + p Iz 2)1/2, and the image in the area of p Ixz < 0 is a mirror image of that in the area of p Ixz > 0. p Ix , p Iy , and p Iz stand for the momentum of the product ions along the x, y, and z axes (See Fig. 2 ). In (e)–(h), p ex and p ey stand for the momentum of the photoelectrons along the x and y axes. The pixel size is set to be 3.0 a.u × 3.0 a.u. in (a)–(d) and 0.01 a.u. × 0.01 a.u. in (e)–(h).

(a)–(d) 2D cuts of 3D momentum distributions of product ions with (a) C2H5OH+, (b) C2H4OH+, (c) CH2OH+, and (d) C2H5 +. (e)–(h) 2D sliced images of the 3D momentum distribution of photoelectrons detected in coincidence with (e) C2H5OH+, (f) C2H4OH+, (g) CH2OH+, and (h) C2H5 +. The laser peak intensity and pulse duration are I 0 = 9 TW/cm2 and τ = 35 fs. The vertical (y) axis is along the laser polarization direction. The unit of the coordinates is the atomic unit (a.u.) in both electron and ion images. On the top axes in (a)–(d), the velocity unit (km/s) is shown as a reference. An atomic unit of the momentum is equal to 1.198 u km/s. In (a)–(d), p Ixz is defined as |p Ixz | = (p Ix 2 + p Iz 2)1/2, and the image in the area of p Ixz < 0 is a mirror image of that in the area of p Ixz > 0. p Ix , p Iy , and p Iz stand for the momentum of the product ions along the x, y, and z axes (See Fig. 2 ). In (e)–(h), p ex and p ey stand for the momentum of the photoelectrons along the x and y axes. The pixel size is set to be 3.0 a.u × 3.0 a.u. in (a)–(d) and 0.01 a.u. × 0.01 a.u. in (e)–(h).

Photoelectron spectra for the formation of C2H5OH+, C2H4OH+, CH2OH+, and C2H5 + (from the top) at the laser peak intensities of I 0 = (a) 9, (b) 17, and (c) 23 TW/cm2 with the pulse duration of τ = 35 fs. The spectra for the C2H5OH+ and C2H5 + formation are multiplied by 0.5 and 10, respectively.

Photoelectron spectra for the formation of C2H5OH+, C2H4OH+, CH2OH+, and C2H5 + (from the top) at the laser peak intensities of I 0 = (a) 9, (b) 17, and (c) 23 TW/cm2 with the pulse duration of τ = 35 fs. The spectra for the C2H5OH+ and C2H5 + formation are multiplied by 0.5 and 10, respectively.

Photoelectron spectra for the formation of C2H5OH+, C2H4OH+, CH2OH+, and C2H5 + (from the top) at the laser peak intensity in the range between I 0 = 17 and 19 TW/cm2. The laser pulse is positively chirped with the pulse duration of (a) 200 and (b) 800 fs, and negatively chirped with the pulse duration of (c) 200 and (d) 800 fs.

Photoelectron spectra for the formation of C2H5OH+, C2H4OH+, CH2OH+, and C2H5 + (from the top) at the laser peak intensity in the range between I 0 = 17 and 19 TW/cm2. The laser pulse is positively chirped with the pulse duration of (a) 200 and (b) 800 fs, and negatively chirped with the pulse duration of (c) 200 and (d) 800 fs.

Energy correlation maps of a photoelectron and a fragment ion detected in coincidence for the formation of (a) CH2OH+ and (b) C2H5 +. The laser peak intensity and pulse duration are I 0 = 9 TW/cm2 and τ = 35 fs. The numbers of the observed events plotted in the maps are 70 857 and 3473 for CH2OH+ and C2H5 +, respectively.

Energy correlation maps of a photoelectron and a fragment ion detected in coincidence for the formation of (a) CH2OH+ and (b) C2H5 +. The laser peak intensity and pulse duration are I 0 = 9 TW/cm2 and τ = 35 fs. The numbers of the observed events plotted in the maps are 70 857 and 3473 for CH2OH+ and C2H5 +, respectively.

Kinetic energy distributions (circles) of CH2OH+ fragment ion when the photoelectron energies are (a) 0.2, (b) 0.8, and (c) 1.5 eV. These distributions are extracted from Fig. 6(a) and reproduced well by a Boltzmann-type distribution, proportional to exp(−E ion/kT) (thick gray lines), where T is a translational temperature of CH2OH+. The fitting results are T(CH2OH+) = 462(6), 284(6), and 465(6) K in (a)–(c), respectively.

Kinetic energy distributions (circles) of CH2OH+ fragment ion when the photoelectron energies are (a) 0.2, (b) 0.8, and (c) 1.5 eV. These distributions are extracted from Fig. 6(a) and reproduced well by a Boltzmann-type distribution, proportional to exp(−E ion/kT) (thick gray lines), where T is a translational temperature of CH2OH+. The fitting results are T(CH2OH+) = 462(6), 284(6), and 465(6) K in (a)–(c), respectively.

Translational temperature of CH2OH+, T(CH2OH+), as a function of E elec when τ = 35 fs and I 0 = (a) 9, (b) 17, and (c) 23 TW/cm2.

Translational temperature of CH2OH+, T(CH2OH+), as a function of E elec when τ = 35 fs and I 0 = (a) 9, (b) 17, and (c) 23 TW/cm2.

Kinetic energy distributions (circles) of C2H5 + fragment ion when the photoelectron energies are (a) 0.2, (b) 0.8, and (c) 1.5 eV. These distributions are extracted from Fig. 6(b) and reproduced well by a Boltzmann-type distribution (see the caption of Fig. 7 ). The fitting results are T(C2H5 +) = 1799(104), 2077(151), and 2077(175) K in (a)–(c), respectively.

Kinetic energy distributions (circles) of C2H5 + fragment ion when the photoelectron energies are (a) 0.2, (b) 0.8, and (c) 1.5 eV. These distributions are extracted from Fig. 6(b) and reproduced well by a Boltzmann-type distribution (see the caption of Fig. 7 ). The fitting results are T(C2H5 +) = 1799(104), 2077(151), and 2077(175) K in (a)–(c), respectively.

Translational temperature of C2H5 +, T(C2H5 +), as a function of E elec when τ = 35 fs and I 0 = (a) 9, (b) 17, and (c) 23 TW/cm2. The large fluctuation in (a) is mainly ascribed to the fact that the number of the accumulated events for the channel yielding C2H5 + is not large enough.

Translational temperature of C2H5 +, T(C2H5 +), as a function of E elec when τ = 35 fs and I 0 = (a) 9, (b) 17, and (c) 23 TW/cm2. The large fluctuation in (a) is mainly ascribed to the fact that the number of the accumulated events for the channel yielding C2H5 + is not large enough.

Translational temperature of CH2OH+ ((a) and (b)) and C2H5 + ((c) and (d)) as a function of E elec when the pulse duration τ is 200 fs ((a) and (c)) and 800 fs ((b) and (d)). The laser peak intensity is kept to be in the range between I 0 = 17 and 19 TW/cm2. Open red circles and solid black squares indicate the results by the positively and negatively chirped pulses, respectively.

Translational temperature of CH2OH+ ((a) and (b)) and C2H5 + ((c) and (d)) as a function of E elec when the pulse duration τ is 200 fs ((a) and (c)) and 800 fs ((b) and (d)). The laser peak intensity is kept to be in the range between I 0 = 17 and 19 TW/cm2. Open red circles and solid black squares indicate the results by the positively and negatively chirped pulses, respectively.

Laser intensity dependence of temperature of (a) CH2OH+ and (b) C2H5 + when τ = 35 fs, and laser pulse duration dependence of temperature of (c) CH2OH+ and (d) C2H5 + when the laser peak intensity is kept to be in the range between I 0 = 17 and 19 TW/cm2. The pulse duration is stretched with the negative (open blue square) and positive (open red circle) chirp rates. The plotted temperatures are obtained through the least-squares fit of the kinetic energy distributions of CH2OH+ and C2H5 + integrated for all the range of E elec.

Laser intensity dependence of temperature of (a) CH2OH+ and (b) C2H5 + when τ = 35 fs, and laser pulse duration dependence of temperature of (c) CH2OH+ and (d) C2H5 + when the laser peak intensity is kept to be in the range between I 0 = 17 and 19 TW/cm2. The pulse duration is stretched with the negative (open blue square) and positive (open red circle) chirp rates. The plotted temperatures are obtained through the least-squares fit of the kinetic energy distributions of CH2OH+ and C2H5 + integrated for all the range of E elec.

Yield ratios of the six product ions, C2H5OH+, C2H4OH+, CH2OH+, C2H5 +, C2H3 +, and CH3 +, when τ = 35 fs and I 0 = (a) 9, (b) 17, and (c) 23 TW/cm2. The ratios are normalized so that the sum of the yields of the six product ions is unity.

Yield ratios of the six product ions, C2H5OH+, C2H4OH+, CH2OH+, C2H5 +, C2H3 +, and CH3 +, when τ = 35 fs and I 0 = (a) 9, (b) 17, and (c) 23 TW/cm2. The ratios are normalized so that the sum of the yields of the six product ions is unity.

Yield ratios of the six product ions, C2H5OH+, C2H4OH+, CH2OH+, C2H5 +, C2H3 +, and CH3 +, when τ = (a) 35, (b) 200, and (c) 800 fs. The laser intensity is kept to be in the range between I 0 = 17 and 19 TW/cm2. In (b) and (c), the laser pulse duration is stretched with the negative (black bar) and positive (gray bar) chirp rate. The ratios are normalized so that the sum of the yields of the six product ions is unity.

Yield ratios of the six product ions, C2H5OH+, C2H4OH+, CH2OH+, C2H5 +, C2H3 +, and CH3 +, when τ = (a) 35, (b) 200, and (c) 800 fs. The laser intensity is kept to be in the range between I 0 = 17 and 19 TW/cm2. In (b) and (c), the laser pulse duration is stretched with the negative (black bar) and positive (gray bar) chirp rate. The ratios are normalized so that the sum of the yields of the six product ions is unity.

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