Ion desorption from molecules condensed at low temperature: A study with electron-ion coincidence spectroscopy combined with synchrotron radiation

This article reviews our recent work on photo-stimulated ion desorption (PSID) from molecules condensed at low temperature. We have used electron-ion coincidence (EICO) spectroscopy combined with synchrotron radiation. The history and present status of the EICO apparatus is described, as well as our recent investigations of condensed H₂O, NH₃, CH₃CN, and CF₃CH₃. Auger electron photoion coincidence (AEPICO) spectra of condensed H₂O at the O:1s ionization showed that H⁺ desorption was stimulated by O:KVV Auger processes leading to two - hole states (normal- Auger stimulated ion desorption (ASID) mechanism). The driving forces for H⁺ desorption were attributed to the electron missing in the O - H bonding orbitals and the effective hole-hole Coulomb repulsion. The normal ASID mechanism was also demonstrated for condensed NH₃. The H⁺ desorption at the 4a₁ ← O(N):1s resonance of both condensed H₂O and condensed NH₃ was found to be greatly enhanced. Based on the AEPICO spectra the following four-step mechanism was proposed: (1) the 4a₁ ← 1s transition, (2) extension of the HO - H (H₂N - H) distance within the lifetime of the (1s)⁻¹(4a1)¹ state, (3) spectator Auger transitions leading to (valence)⁻²(4a₁)¹ states, and (4) H⁺ desorption. The enhancement of the H⁺ desorption yield was attributed to the repulsive potential surface of the (1s) - 1(4a₁)¹ state. At the 3p ← O:1s resonance of condensed H₂O, on the other hand, the H⁺ yield was found to be decreased. The AEPICO spectra showed that the H⁺ desorption was stimulated by spectator Auger transitions leading to (valence)⁻²(3p)¹ states. The decrease in the H⁺ yield was attributed to a reduction in the effective hole-hole Coulomb repulsion due to shielding by the 3p electron. Photoelectron photoion coincidence (PEPICO) spectra of condensed H₂O showed that the core level of the surface H₂O responsible for the H⁺ desorption was shifted by 0.7 eV from that of the bulk H₂O. The H⁺ desorption from condensed CH₃CN was also investigated. In a study of condensed CF₃CH₃ using PEPICO spectroscopy, site-specific ion desorption was directly verified; that is, H⁺ and CH₃⁺ desorption was predominant for the C:1s photoionization at the -CH₃ site, while C₂Hn⁺, CFCHm⁺, and CF₃⁺ desorption was predominantly induced by the C:1s photoionization at the -CF₃ site. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a powerful tool for studying PSID of molecules condensed at low temperature

Filter bandwidths using the (2,0)_<em>n</em> absorption profiles as functions of helium column density

<p><strong>Figure 6.</strong> Filter bandwidths using the (2,0)_<em>n</em> absorption profiles as functions of helium column density. For <em>n</em> = 2 (red) the two half-widths (dashed lines) are shown along with the full-width at half-maximum. Only the FWHMs are shown for <em>n</em> = 3 (blue), <em>n</em> = 4 (green), and <em>n</em> = 5 (purple). The dashed vertical lines show the maximum column density used in this work (68 <b>×</b> 10¹⁷ cm⁻²) and the column density which would correspond to 2 m of gas at atmospheric pressure (5 <b>×</b> 10²¹ cm⁻²). The solid black circles show the bandwidths obtained in the spectra plotted in figures <a href="http://iopscience.iop.org/0953-4075/46/16/164021/article#jpb465316f3" target="_blank">3</a> and <a href="http://iopscience.iop.org/0953-4075/46/16/164021/article#jpb465316f4" target="_blank">4</a>.</p> <p><strong>Abstract</strong></p> <p>Using the third harmonic of the FEL radiation from the SPring-8 compact SASE (self-amplified stimulated emission) source SCSS we have studied the effects on SASE pulses with central wavelengths near 20 nm due to passage through a helium gas cell. The positions of zero ionization cross-section close to wavelengths corresponding to double-excitations allow operation as an efficient wavelength filter, with effectively 100% transmitted peak intensity until the Doppler-broadening limit is reached. We discuss how the time profile of the SASE pulses is affected, and discuss potential applications.</p

Single-shot simulation at a column density of 68 <b>×</b> 10¹⁷ cm⁻² near the <em>n</em> = 3 and <em>n</em> = 4 resonances

<p><strong>Figure 5.</strong> Single-shot simulation at a column density of 68 <b>×</b> 10¹⁷ cm⁻² near the <em>n</em> = 3 and <em>n</em> = 4 resonances. The upper panel shows the simulated SASE wavelength profile (red), the transmission of the gas cell (black, same <em>x</em>-axis), the phase due to absorption (black, right-hand <em>y</em>-axis), and the transmitted wavelength profile (blue). The lower panel shows the time profile of the electric field before (red) and after (blue) the gas cell.</p> <p><strong>Abstract</strong></p> <p>Using the third harmonic of the FEL radiation from the SPring-8 compact SASE (self-amplified stimulated emission) source SCSS we have studied the effects on SASE pulses with central wavelengths near 20 nm due to passage through a helium gas cell. The positions of zero ionization cross-section close to wavelengths corresponding to double-excitations allow operation as an efficient wavelength filter, with effectively 100% transmitted peak intensity until the Doppler-broadening limit is reached. We discuss how the time profile of the SASE pulses is affected, and discuss potential applications.</p

Absorption cross-section σ (red, left-hand <em>y</em>-axis), and transmission (right-hand <em>y</em>-axis) for different column densities (/10¹⁷ cm⁻²)

<p><strong>Figure 1.</strong> Absorption cross-section σ (red, left-hand <em>y</em>-axis), and transmission (right-hand <em>y</em>-axis) for different column densities (/10¹⁷ cm⁻²).</p> <p><strong>Abstract</strong></p> <p>Using the third harmonic of the FEL radiation from the SPring-8 compact SASE (self-amplified stimulated emission) source SCSS we have studied the effects on SASE pulses with central wavelengths near 20 nm due to passage through a helium gas cell. The positions of zero ionization cross-section close to wavelengths corresponding to double-excitations allow operation as an efficient wavelength filter, with effectively 100% transmitted peak intensity until the Doppler-broadening limit is reached. We discuss how the time profile of the SASE pulses is affected, and discuss potential applications.</p

Detail showing the transmission near (2,0)₄ for a column density of ~68 <b>×</b> 10¹⁷ cm⁻²

<p><strong>Figure 4.</strong> Detail showing the transmission near (2,0)₄ for a column density of ~68 <b>×</b> 10¹⁷ cm⁻². The series limit converging on He⁺(<em>n</em> = 2) is at 18.96 nm. The inset shows the single-shot spectra with the highest peak intensities at the positions of the He** absorption profiles along with the calculated average transmission (black trace).</p> <p><strong>Abstract</strong></p> <p>Using the third harmonic of the FEL radiation from the SPring-8 compact SASE (self-amplified stimulated emission) source SCSS we have studied the effects on SASE pulses with central wavelengths near 20 nm due to passage through a helium gas cell. The positions of zero ionization cross-section close to wavelengths corresponding to double-excitations allow operation as an efficient wavelength filter, with effectively 100% transmitted peak intensity until the Doppler-broadening limit is reached. We discuss how the time profile of the SASE pulses is affected, and discuss potential applications.</p

Detail showing the transmission near (2,0)₃ for a column density of ~68 <b>×</b> 10¹⁷ cm⁻²

<p><strong>Figure 3.</strong> Detail showing the transmission near (2,0)₃ for a column density of ~68 <b>×</b> 10¹⁷ cm⁻². The inset shows the two single-shot spectra with the highest transmitted intensities at the wavelengths of zero cross-section (red, blue), along with the calculated average transmission (black).</p> <p><strong>Abstract</strong></p> <p>Using the third harmonic of the FEL radiation from the SPring-8 compact SASE (self-amplified stimulated emission) source SCSS we have studied the effects on SASE pulses with central wavelengths near 20 nm due to passage through a helium gas cell. The positions of zero ionization cross-section close to wavelengths corresponding to double-excitations allow operation as an efficient wavelength filter, with effectively 100% transmitted peak intensity until the Doppler-broadening limit is reached. We discuss how the time profile of the SASE pulses is affected, and discuss potential applications.</p