Multiply ionization of diethyl ether clusters by 532nm nanosecond laser: The influence of laser intensity and the electron energy distribution

Intensity dependence of multiply charged atomic ions from argon clusters in moderate nanosecond laser fields

We report the laser intensity dependence of multiply charged atomic ions (MCAIs) Arⁿ⁺ with 2 ≤ n ≤ 8 from argon clusters in focused nanosecond laser fields at 532 nm. The laser field, in the range of 10¹¹-10¹² W/cm², is insufficient for optical field ionization but is adequate for multiphoton ionization. The MCAI sections of the mass spectra for clusters containing 3700 and 26 000 atoms are dominated by Arⁿ⁺ with 7 ≤ n ≤ 9, extending to Ar¹⁴⁺. While the distributions of the MCAIs remain largely constant throughout the intensity range of the laser, the abundance of Ar⁺ relative to the abundances of the MCAIs increases dramatically with increasing laser intensity. Consequently, exponential fittings of the yields result in a larger exponent for Ar⁺ than for MCAIs, and the exponents of MCAIs with 2 ≤ n ≤ 8 are similar, with only slight variations for different charge states. The width of the arrival time and, hence, the corresponding kinetic energy of Ar⁺ also increases with increasing laser intensities, while the width of the arrival time of MCAIs remains constant throughout the range of measurements. These results call for more detailed theoretical investigations in this regime of laser-matter interactions.

Volume averaging effect in nonlinear processes of focused laser fields

We report theoretical derivations and experimental results on the volume averaging effect of nonlinear processes in focused laser fields. This effect is considered detrimental in revealing the intensity dependence of a nonlinear process, caused by the intensity variation across the sampled volume of a focused laser. Following the treatment in the literature, we prove that if the signal dependence can be expressed as a simple power function of the laser intensity and if the detection region encompasses effectively the whole volume, volume average does not affect the final conclusion on the derived exponent. However, to reveal the detailed saturation effect of a multi-photon process, intensity selective scans involving spatial filters and displacement of the laser focus (z-scan) are required. Moreover, to fully capture the dependence of the signal on the variation of the laser intensity, the degree of spatial discrimination and the corresponding range of the z-scan need to be modeled carefully. Limitations in the dynamic range of the detector or the laser power, however, can thwart the desired scan range, resulting in erroneous fitting exponents. Using our nanosecond laser with a non-ideal Gaussian beam profile based on multiphoton ionization of argon atoms from a collimated molecular beam and from ambient argon gas, we report experimental measurements of the beam waist and Rayleigh range and compare the experimental intensity dependence of Ar⁺ with theoretical values. Agreements between theory and experiment are remarkable.

Production of Multiply Charged Argon Ions in Moderate Nanosecond Laser Fields: An Open Question or a Forgone Conclusion?

We reply to the Viewpoint by Vatsa and Mathur on our publication reporting the observation of multiply charged atomic ions from argon clusters doped with aromatic chromophores in a moderate nanosecond laser field. Vatsa and Mathur raised three concerns about the proposed explanation and offered additional ideas for the reported process. We agree with some of their concerns and welcome the addition of information, and we also clarify a few misunderstandings of our intention, perhaps caused by our implicit assumption of contextual relations. While the experimental results are indisputable, the interpretation is still a topic of debate, subject to further experimental investigations and theoretical modeling.

Coulomb Explosion in Nanosecond Laser Fields

We report experimental observations of Coulomb explosion using a nanosecond laser at 532 nm with intensities less than 10¹² W/cm². We observe multiply charged atomic ions Arⁿ⁺ (1 ≤ n ≤ 7) and Cⁿ⁺ (1 ≤ n ≤ 4) from argon clusters doped with molecules containing aromatic chromophores. The yield of Arⁿ⁺ depends on the size of the cluster, the number density, and the photostability of the dopant. We propose that resonant absorption of Ar_N⁺ achieves a high degree of ionization, and the highly positively charged cluster has the capability to strip electrons from the evaporating Ar⁺ on the surface of the cluster prior to Coulomb explosion, forming Arⁿ⁺.

Following laser induced changes of plant phenylpropanoids by Raman microscopy

Raman microscopy is a powerful imaging technique for biological materials providing information about chemistry in context with microstructure. A 532 nm laser is often used as excitation source, because high spatial resolution and signal intensity can be achieved. The latter can be controlled by laser power and integration time, whereby high power and long times give good signal to noise ratio. However, most biological materials absorb in the VIS range and fluorescence masking the signal or even sample degradation might be hindering. Here, we show that on lignified plant cell walls even very short integration times and low laser powers induce a change in the ratio of the lignin bands at 1660 and 1600 cm-1. Time series on lignin model compounds revealed this change only in aromatic molecules with two OH-groups, such as coniferyl alcohol. Therefore, we conclude that monolignols are present in the cell wall and responsible for the observed effect. The solvent selectivity of the changes points to a laser induced polymerization process. The results emphasize how crucial careful adjustment of experimental parameters in Raman imaging of biological materials is and show the potential of time series and repeated imaging to get additional insights (e.g. monolignols).

(TMT)n clusters as dilute debris-free tin source for generation of multiply charged tin ions-of relevance in extreme ultraviolet (EUV) lithography, under intense laser irradiation

RATIONALE

Multiply charged tin ions ([Sn]8+ to [Sn]13+ ) are considered as ideal-emitters at extreme ultraviolet (EUV) wavelength ~ 13.5 nm, pertinent to advanced micro-electronic device fabrication. Solid tin targets have been widely explored for the generation of these ions, but debris generation has restricted their utilization. Tin-containing molecular clusters have, however, the potential to act as dilute and debris-free sources for the generation of tin ions, as investigated in the present study.

METHODS

Efficient coupling of laser energy with clusters has been utilized for the generation of multiply charged tin ions. The ions generated have been characterized using time-of-flight mass spectrometry. By varying experimental parameters, the yield of multiply charged tin ions has been manipulated, for tetramethyltin (TMT) clusters. In addition, the energetic electrons and photons liberated upon laser-cluster interaction have been probed utilizing an in-house developed retarding field analyser.

RESULTS

Interaction of tetramethyltin clusters with 1064 nm picosecond laser pulses of intensity ~ 1013 W/cm2 leads to the generation of multiply charged tin ions (up to [Sn]14+ ) and carbon ions (up to [C]4+ ). Efficient laser-cluster interaction is evident from the generation of multiply charged tin ions with ionization energy ~ 282 eV ([Sn]14+ ). By varying the experimental conditions, the distribution of multiply charged tin ions can be influenced to obtain significant relative ion yield of multiply charged tin ions from [Sn]8+ to [Sn]13+ .

CONCLUSIONS

Tetramethyltin clusters have the potential to act as dilute and debris-free source for EUV lithographic applications, in contrast to bulk tin targets. The inherent properties of clusters, such as higher local density and the pulsed nature of the cluster source, are appropriate for EUV lithographic applications.

Mass spectrometric and charge density studies of organometallic clusters photoionized by gigawatt laser pulses

Clusters on exposure to nanosecond laser pulses of gigawatt intensity exhibit a variety of photo-chemical processes such as fragmentation, intracluster reaction, ionization, Coulomb explosion, etc. Present article summarizes the experimental results obtained in our laboratory utilizing time-of-flight mass spectrometer which deal with one such aspect of cluster photochemistry related to generation of multiply charged atomic ions upon excessive ionization of cluster constituents (Coulomb explosion) at low intensity laser field (∼10⁹  W/cm² ). To understand the mechanism of laser-cluster interaction, laser as well as cluster parameters were varied. Mass spectrometric studies were carried out at different laser wavelength as well as varying the nature of cluster constituents, backup pressure, nozzle diameter, etc. In addition, charge density measurements were also preformed to get information about the total number of ions generated upon laser-cluster interaction as a function of laser wavelength. In case of pure molecular clusters, the charge state of atomic ions as well as charge density was observed to enhance with increasing laser wavelength, signifying efficient coupling of the cluster medium with nanosecond laser pulse at longer wavelength. While in case of clusters doped with species having comparatively lower ionization energy, the efficiency of laser-cluster interaction was less, in contrast to studies carried out using femtosecond lasers. Results obtained in the present work have been rationalized on the basis of proposed three-stage cluster ionization mechanism, that is, multiphoton ionization ignited-inverse Bremsstrahlung heating and electron ionization. © 2015 Wiley Periodicals, Inc. Mass Spec Rev. 36:188-212, 2017.

Ionisation of methyl iodide clusters using nanosecond laser pulses: detection of multiply charged positive ions, negative ions and energetic electrons

Methyl iodide clusters have been ionised using laser pulses of intensity ∼10⁹ W cm⁻² at 266, 355, 532 and 1064 nm and the ions produced in the laser–cluster interaction were analysed with a time-of-flight mass spectrometer.

Multiphoton ionization and Coulomb explosion of C2H5Br clusters: a mass spectrometric and charge density study

Using time-of-flight mass spectrometry (TOFMS), laser-induced photochemistry of ethyl bromide clusters has been investigated at three different wavelengths (viz. 266, 355 and 532  nm) utilizing nanosecond laser pulses of ~5 × 10(9)  W/cm(2). An interesting finding of the present work is the observation of multiply charged atomic ions of carbon and bromine at 355 and 532  nm, arising from the Coulomb explosion of (C(2)H(5)Br)(n) clusters. At 266  nm, however, the (C(2)H(5)Br)(n) clusters were found to exhibit the usual multiphoton dissociation/ionization behaviour. The TOFMS studies are complemented by measuring the total charge density of the ionized volume at 266, 355 and 532  nm, using the parallel plate method, and the charge densities were found to be ~2 × 10(9), 6 × 10(9) and 2 × 10(11)  charges/cm(3), respectively. The significantly higher charge density and the presence of energetic, multiply charged atomic ions at 532  nm are explained by the higher ponderomotive energy of the 532  nm photon, coupled with the Coulomb stability of the residual multiply charged ethyl bromide clusters generated upon laser irradiation, due to their larger effective cluster size at 532  nm than at 355 and 266  nm.