Linear and tri-branched copolymers for two-photon absorption and two-photon fluorescent materials

The one- and two-photon properties of linear (M2) and tri-branched (M3) copolymers with triphenylamine and cyano groups in the main chain were experimentally investigated. Open-aperture z-scan experiments were performed with 1 kHz, 120 fs, and 800 nm Ti:sapphire laser pulses to measure the two-photon absorption cross sections. The two-photon cross sections of M2 and M3 were determined to be 0.304 and 1.441×10⁻²⁰ cm⁴/Gw per repeating unit, respectively. In a CHCl₃ solution, M2 and M3 emit strong frequency up-converted fluorescence under the excitation of 120 fs pulses at 800 nm with the peaks located at 561 and 542 nm.     

Aerosol sample preparation methods for X-ray diffractive imaging: Size-selected spherical nanoparticles on silicon nitride foils

We are developing aerosol generating and processing methods for X-ray analyses of nanoscale materials using conventional synchrotron radiation sources and using the newly operational soft X-ray free-electron laser in Hamburg (FLASH) at the Deutsches Elektronen Synchrotron. Charge-reduced electrospray, differential mobility analysis and an electrostatic precipitator were used to prepare samples consisting of size-monodisperse spherical nanoparticles deposited on 20 nm thick silicon nitride foils supported by silicon frames. Ninety-seven and 102 nm diameter spheres were selected from a broader distribution of 98 nm spheres using differential mobility. We measured the size distribution of the spheres using forward scattering from 1.65 nm light at the Advanced Light Source (ALS) in Lawrence Berkeley National Laboratory and scanning electron microscopy (SEM). The full-width half maximum (FWHM) of the size distribution of the size-selected spheres was as narrow as 5.4 nm when measured by SEM, as compared to 16 nm for the non-size-selected distribution. Forward scattering measurements of the 97 nm diameter size-selected spheres fit a size distribution with a FWHM of 4 nm and allowed us to validate the methodology for use in future diffraction imaging experiments at FLASH.     

Aerosolization of colloidal nanoparticles by a residual-free atomizer

Abstract

A new residual-free atomizer was designed to transfer colloidal nanoparticles measuring less than 100?nm into aerosol phase. Miniaturization of droplet size distribution successfully reduced background aerosol concentration of particles sized greater than 2.5?nm to 400 particles·mL^?1 of gas, which corresponded to an NaCl-equivalent impurity concentration of less than 100?ppb. Direct injection of colloid suspension enabled precise control of aerosol number concentrations by colloidal concentration (10⁵–10¹¹ particles·mL^?1 of liquid). Correlations between the size distributions of colloid and aerosol were also investigated using aqueous suspensions of the standard nanoparticles sized 10–100?nm. It was found that the aerosol size distribution was in very good agreement (i.e., less than 1?nm accuracy) with that measured by scanning electron microscopy.

Copyright © 2020 American Association for Aerosol Research     

Using Multiphoton Absorption with High-Intensity Lasers to Heat Transparent Liquids

Most pure, organic liquids are remarkably transparent materials, even when exposed to high laser intensities. At very high laser intensities, however, ∼10⁹–10¹⁰ W/cm² and under the appropriate conditions, liquids can absorb extremely strongly due to the mechanism of multiphoton absorption, which results in ionization and dissociation of the liquid molecules. Strong absorption results in significant heating and temperature increases. Multiphoton absorption thus provides a new mechanism for heating otherwise transparent liquids. Furthermore, the thermal energy is deposited in a very thin layer near the surface of the liquid, which is much thinner than the classical penetration depth. A thermal model based on multiphoton absorption is developed, and results are presented for heating liquid water with laser pulses from the popular Nd: YAG laser at a wavelength of 266 nm.     

Effect of laser intensity on yield and physical characteristics of single wall carbon nanotubes produced by the Nd:YAG laser vaporization method

We report on the effect of the Nd:YAG laser intensity on diameter distribution, yield and physical characteristics of single-wall carbon nanotubes (SWNTs) while comparing three different laser configurations (namely: (i) single 532 nm pulse; (ii) single 1064 nm pulse; and (iii) 532 nm followed by the 1064 nm double pulse). The carbon SWNTs were synthesized at a furnace temperature of 1150 °C and characterized by means of laser micro-Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). Regardless of the laser configuration used, it is found that both the yield and the structural characteristics of the SWNTs are highly sensitive to the laser intensity. Indeed, by combining Raman analyses together with HRTEM observations we were able to point out the existence of an optimal laser intensity which leads not only to the highest yield of SWNTs and the largest bundles but also to the lowest level of amorphous and, or disordered sp² carbon in the deposits. While the optimal laser intensity was found to increase from 1.7 to 2.9×10⁹ W/cm² when the laser wavelength is changed from 1064 to 532 nm, the double pulse configuration offered a larger process latitude since high yield of SWNTs was obtained over the (0.8–3.5)×10⁹ W/cm² laser intensity range centered around the optimal value of 2.3×10⁹ W/cm². Moreover, it is shown that the increase of the laser intensity (from 0.5 to 5.6×10⁹ W/cm²) favors the growth of large nanotubes (1.4 nm-diam.) to the detriment of smaller ones (1.1 nm-diam.). A tendency to form larger nanotubes was also observed when increasing the furnace temperature from 1000 to 1150 °C. Finally, the laser intensity effect is interpreted in terms of near-surface or deep laser energy absorption in the graphite target.     

Aircraft Instrument for Comprehensive Characterization of Aerosol Optical Properties,Part I: Wavelength-Dependent Optical Extinction and Its Relative Humidity Dependence Measured Using Cavity Ringdown Spectroscopy

High-quality in situ observations of aerosol particle optical properties, namely extinction, scattering, and absorption, provide important information needed to constrain the role of aerosols in the climate system. This paper outlines the design and performance of an aircraft instrument utilizing cavity ringdown spectroscopy for the measurement of aerosol extinction. The 8-channel cavity ringdown spectrometer measures extinction at multiple wavelengths (405, 532, and 662 nm) and at multiple relative humidities (e.g., 10%, 70%, and 95%). Key performance characteristics include a 1-s detection limit better than 0.1 Mm^?1, accuracy of <2% for dry aerosol measurements, and a 1-s precision better than 40% for extinction levels of >10 Mm^?1. Laboratory and field data demonstrate that the 1-s precision is limited by the statistics of aerosol particles in the laser beam rather than the precision of the extinction measurement per se. The measurement precision improves with averaging to 5% at 60 s for extinction levels of >10 Mm^?1. Field data collected during a recent airborne campaign in California, which involved eighteen research flights during May and June 2010, are used to demonstrate the in-flight performance of new instrument.     

Synthesis and nonlinear optical characterization of copolymers containing alternating 3,4‐dialkoxythiophene and (1,3,4‐oxadiazolyl)benzene units

We report the synthesis and linear and third‐order nonlinear optical (NLO) characterization of two novel copolymers containing alternating 3,4‐dialkoxythiophene and 1,4‐bis(1,3,4‐oxadiazolyl)benzene units. The copolymers were synthesized with a precursor polyhydrazide route. Both copolymers exhibited fluorescence around 430 nm under the irradiation of UV light. The NLO measurements were made with the single‐beam Z‐scan technique with Nd:YAG nanosecond laser pulses at 532 nm. The nonlinear refractive index of the investigated copolymers was negative, and the magnitude was as high as 10^?10 esu. The samples exhibited strong reverse saturation absorption and very good optical limiting properties at the wavelength used. The concentration dependence of third‐order NLO parameters was studied. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Time-resolved spectroscopy of infrared active vibrations in 2,5-dioctyloxy poly(phenylene vinylene) films

We report on picosecond time resolved spectroscopy of photogenerated infrared active vibrations in thin films of 2,5-dioctyloxy poly(phenylene vinylene). We excited the films by ?4 ps long pulses of 565 nm laser light with 2×10¹³ photons/cm² per pulse and repetition rate of 76 MHz. We then followed the temporal evolution of the infrared active vibrational (IRAV) spectrum using a subsequent, variably delayed, weak tunable IR probe pulses of similar temporal duration. Under these conditions, we show clear spectroscopic evidence for photogenerated infrared active vibrations at times which are shorter than our temporal resolution (<4 ps). We suggest that the transient IRAV absorption is due to secondary polarons formation following exciton dissociation.     

Generating mechanical pulses by the laser blasting of explosive coatings

For strength tests of materials and structures, a method of generating mechanical pulses over a large area (1 m²) is proposed on the basis of the laser blasting of explosive coatings. Compositions with record high levels of sensitivity (4·10^–3-4·10^–2 J/cm²) to a single laser pulse are developed, as well as technologies for producing coatings of these compositions on various materials. This approach yields submicrosecond load pulses of density 0.08–1.0 kPa sec.Mining Institute, 3200600 Dnepropetrovsk. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 2, pp. 106–111 March–April, 1994.     

An electrostatic sensor for the continuous monitoring of particulate air pollution

We developed and evaluated a particulate air pollution sensor for continuous monitoring of size resolved particle number, based on unipolar corona charging and electrostatic detection of charged aerosol particles. The sensor was evaluated experimentally using combustion aerosol with particle sizes in the range between approximately 50 nm and several microns, and particle number concentrations larger than 10¹⁰ particles/m³. Test results were very promising. It was demonstrated that the sensor can be used in detecting particle number concentrations in the range of about 2.02×10¹¹ and 1.03×10¹² particles/m³ with a response of approximately 100 ms. Good agreement was found between the developed sensor and a commercially available laser particle counter in measuring ambient PM along a roadside with heavy traffic for about 2 h. The developed sensor proved particularly useful for measuring and detecting particulate air pollution, for number concentration of particles in the range of 10⁸ to 10¹² particles/m³.     

Characterization of an Automated,Water-Based Expansion Condensation Nucleus Counter for Ultrafine Particles

The design and experimental characterization of a condensation nucleus counter (CNC) is presented. The counter produces supersaturation by means of fast volume-controlled adiabatic expansion. The aerosol number concentration is derived from observing scattered laser light in the forward direction under a solid angle between 1.1° and 4.4° over the full annular sector. The number concentration is derived by application of Mie theory from the characteristic pattern in the temporal evolution of the detected signal during the droplet growth process. The equation for calculation of the aerosol number density by this method is presented. Theoretical considerations for the smallest aerosol particles that can be activated indicate a lower size cut-off between 2.5 and 3.0 nm. Model calculations of the expected Mie scatter signal during expansion agree very well with the experimental observations. The Expansion-CNC can be operated fully automated under computer (PC) control in 10-second sample cycles. For characterization it is compared with a TSI 3025A Ultrafine-CPC (TSI UCPC) for measurements of monodisperse sodium chloride and sulfuric acid aerosol particles, indicating good agreement between the two counters down to particle sizes as low as 3.5 nm under laboratory conditions. In addition, ambient aerosol measurements in urban air show excellent agreement with simultaneous TSI UCPC measurements for particle number concentrations ranging from roughly 50 cm^{? 3} to 130000 cm^{? 3}.     

Single Charging of Nanoparticles by UV Photoionization at High Flow Rates

The feasibility of UV photoionization for single unipolar charging of nanoparticles at flow rates up to 100 l· min ^?1 is demonstrated. The charging level of the aerosol particles can be varied by adjusting the intensity of the UV radiation. The suitability of a UV photocharger followed by a DMA to deliver monodisperse nanoparticles at high aerosol flow rates has been assessed experimentally in comparison to a radioactive bipolar charger ( ⁸⁵ Kr, 10 mCi). Monodisperse aerosols with particle sizes in the range of 5 to 25 nm and number concentrations between 10 ⁴ and 10 ⁵ cm ^?3 have been obtained at flow rates up to 100 l· min ^?1 with the two aerosol chargers. In terms of output particle concentration, the UV photoionizer performs better than the radioactive ionizer with increasing aerosol flow rate. Aerosol charging in the UV photoionizer is described by means of a photoelectric charging model that relies on an empirical parameter and of a diffusion charging model based on the Fuchs theory. The UV photocharger behaved as a quasi-unipolar charger for polydisperse aerosols with particles sizes less than 30 nm and number concentrations ～10 ⁷ cm ^?3 . Much reduced diffusion charging was observed in the experiments, with respect to the calculations, likely due to ion losses onto the walls caused by unsteady electric fields in the irradiation region.     

Characterization and demonstration of a black carbon aerosol mimic for instrument evaluation

This study describes the characterization of a H₂O-dispersible, highly-absorbing carbonaceous nanomaterial that mimics the morphological and spectroscopic properties of aged black carbon aerosol (BC). When atomized from aqueous suspension, the material forms particles with a collapsed morphology resembling aged soot or BC. The material is >90 percent elemental carbon and has a mass absorption coefficient (MAC) and spectral dependence consistent with BC values published in the literature. The MAC at a wavelength of 532?nm decreased monotonically from 8.5 to 5.8?m² g^?1 for aerosol with mobility diameters between 150?nm to 500?nm. The single scatter albedo (SSA) at wavelengths of 405?nm and 660?nm was a function of both wavelength and mobility diameter and increased from 0.1 to 0.4 with mobility diameters between 150?nm to 400?nm. The Ångström absorption exponent (AAE) between λ?=?405?nm and 780?nm decreased monotonically from 1.4 to 0.6 for aerosol with mobility diameters between 150?nm to 400?nm. We demonstrate that this material can be used for fast, efficient calibration of aerosol photoacoustic spectrometers and for evaluation of spectroscopic-based measurements of aerosol mass concentration using in-situ photoacoustic spectroscopy (PAS) and filter-based light attenuation measurements for up 50?µg m^?3, enabling inter-method and inter-laboratory instrument comparison.

Copyright © 2019 American Association for Aerosol Research     

Aerosol Gelation: Synthesis of a Novel,Lightweight, High Specific Surface Area Material

We demonstrate that an aerosol can gel. This gelation is then used for a one-step method to produce an ultralow density porous carbon material. This material is named an aerosol gel because it is made via gelation of particles in the aerosol phase. The carbon aerosol gels have high specific surface area (200–350 m ² /g), an extremely low density (2.5–5.0 mg/cc) and a high electrical conductivity, properties similar to conventional aerogels. The primary particles of the carbon aerosol gels are highly crystalline with a narrow (002) graphitic X-ray diffraction peak. Key aspects to form a gel from an aerosol are large volume fraction, ca. 10 ^?4 or greater, and small primary particle size, 50 nm or smaller, so that the gel time is fast compared to other characteristic times.     

Deposition of functionalized single wall carbon nanotubes through matrix assisted pulsed laser evaporation

Single-wall carbon nanotubes (SWCNTs) functionalized with oxygen-containing groups were deposited onto glass substrates by matrix assisted pulsed laser evaporation (MAPLE). The experiments were performed by subjecting ultraviolet laser pulses (KrF¹ excimer laser, 248 nm wavelength) to frozen SWCNT-toluene targets placed in a parallel plane a few cm in front of the substrate. The morphology, structure, and chemical composition of the deposited materials were studied through atomic force microscopy, high resolution transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The influence of the laser fluence on the material structure was investigated. The results indicate that the functionalized SWCNTs can be transferred by MAPLE at low laser fluences without the alteration of the structure of the initial material used as targets in MAPLE experiments. An increase of the fluence leads to the decomposition of the functional groups, mainly carboxylic acid groups, without degradation of the SWCNT structure whereas, at the highest fluences, the amorphization and even coalescence of the carbon nanotubes takes place.     

Charge neutralization of submicron aerosols using surface-discharge microplasma

In this study we investigated the charging characteristics of a novel aerosol neutralizer (Surface-discharge Microplasma Aerosol Charger; SMAC) based on the dielectric barrier discharging. The surface discharge was induced by supplying positive and negative DC pulses with a pair of micro-structured electrodes. We confirmed the occurrence of the surface discharge by measuring the microdischarge current, and evaluated the charging performance of the SMAC as a particle neutralizer by measuring the penetration efficiency, neutralizing probability, and charge distribution for particles in the size range of 10–200 nm. The SMAC was found to obtain a particle penetration exceeding 90% for the whole particle size range. The neutral fraction obtained by the SMAC showed good agreement with a bipolar diffusion charging theory and the fraction obtained by an ²⁴¹Am radioactive source when the SMAC was optimized for aerosol neutralization with the offset voltage control. The charge distributions of negatively and positively charged particles by the SMAC and the ²⁴¹Am neutralizer were in good agreement also. The charge balance of positive and negative particles obtained by the SMAC was effectively controlled by adjusting the offset voltage on each electrode. This is the first study to demonstrate the successful use of dielectric barrier surface discharge to bring particles of 10–200 nm to an equilibrium charging state in a controllable manner.     

Fluorescence Particle Counter for Detecting Airborne Bacteria and Other Biological Particles

We have constructed a laser-based particle counter that detects the fluorescence, as well as the elastic scattering, from individual airborne particles as they traverse a laser beam. This fluorescence particle counter (FPC) can detect fluorescence from μm-sized Bacillus subtilis spore agglomerates when illuminated with intense light at 488 nm from an argon ion laser, either ～ 0.7 kW cm^?2 extracavity or ～ 50 kW cm^?2 intracavity. We suspect that flavins in the spores are the molecules primarily responsible for the fluorescence, because the peak fluorescence emission of the biological materials at this excitation wavelength is in the range 530–550 nm, which is characteristic of flavins. Fluorescence from kaolin, hematite, and polystyrene particles was not detectable; the lack of fluorescence indicates that the FPC may be able to differentiate between biological and nonbiological aerosols. The FPC samples aerosol-laden air at a rate of ～ 1 mL s^?1, and is capable of measuring aerosol concentrations up to several thousand per milliliter. The FPC may be helpful in detecting and characterizing airborne bacteria and other airborne particles of biological origin.     

Dual-cavity spectrometer for monitoring broadband light extinction by atmospheric aerosols

Abstract

Atmospheric Aerosols affect Earth’s climate directly by scattering and absorbing solar radiation. In order to study the optical properties of aerosols, we developed a broadband cavity-enhanced spectrometer that uses a supercontinuum laser source and a compact spectrometer, to measure simultaneously the extinction coefficient of aerosols over a broad wavelength region from 420 to 540?nm. The system employs a dual cavity approach with a reference and a sample cavity, accounting for changes in gases background and for laser spectral and intensity fluctuations. We tested the system with aerosolized salt particles and polystyrene latex spheres. We performed calculations using Mie theory and found good agreement with the measured extinction. We also found that the extinction coefficient of non-absorbing aerosol favorably compares with the scattering coefficient measured by a nephelometer. Finally, we generated soot particles and found an extinction Ångström exponent in good agreement with values reported in the literature. Wavelength dependent detection limits (1σ) for the instrument at 5?nm wavelength resolution and for an integration time of ～10?min were found to be in the range ～5?Mm^?1 to 13?Mm^?1. The broadband dual-cavity extinction spectrometer is simple and robust and might be particularly useful for laboratory measurements of the extinction coefficient of brown carbon aerosol. The laboratory tests suggest that the prototype is promising for future developments of a field-deployable instrument.

Copyright © 2020 American Association for Aerosol Research     

Investigations of SP-AMS Carbon Ion Distributions as a Function of Refractory Black Carbon Particle Type

The soot particle aerosol mass spectrometer (SP-AMS) instrument combines continuous wave laser vaporization with electron ionization aerosol mass spectrometry to characterize airborne, refractory black carbon (rBC) particles. The laser selectively vaporizes absorbing rBC-containing particles, allowing the SP-AMS to provide direct chemical information on the refractory and non-refractory chemical components, providing the potential to fingerprint various rBC particle types. In this study, SP-AMS mass spectra were measured for 12 types of rBC particles produced by industrial and combustion processes to explore differences in the carbon cluster (C_n⁺) mass spectra. The C_n⁺ mass spectra were classified into three categories based on their ion distributions, which varied with rBC particle type. The carbon ion distributions were investigated as a function of laser power, electron ionization (on/off), and ion charge (positive or negative). Results indicate that the dominant positive ion-formation mechanism is likely the vaporization of small, neutral carbon clusters followed by electron ionization (C₁⁺ to C₅⁺). Significant ion signal from larger carbon cluster ions (and their fragment ions in the small carbon cluster range), including mid carbon (C₆⁺ to C₂₉⁺) and fullerene (greater than C₃₀⁺) ions, were observed in soot produced under incomplete combustion conditions, including biomass burning, as well as in fullerene-enriched materials. Fullerene ions were also observed at high laser power with electron ionization turned off, formed via an additional ionization mechanism. We expect this SP-AMS technique to find application in the identification of the source and atmospheric history of airborne ambient rBC particles.

Copyright 2015 American Association for Aerosol Research     

Ten years (1986–1995) of lidar observations of temporal and vertical structure of stratospheric aerosols over Siberia

The data of laser sensing of the vertical aerosol distribution obtained at Siberian Lidar Station of the Institute of Atmospheric Optics, Tomsk (56.5°N, 85.1°E), Russia in 1986–1995 are analyzed. The data are presented in the form of the scattering ratio R(H), the aerosol backscattering coefficient β^a_π(H), and the integrated aerosol backscattering coefficient between 15 and 30 km Σβ^a_π at the wavelength λ=532 nm. During the period from summer 1986 to summer 1991, Σβ^a_π varied in the range 0.8×10^-4–3×10^-4 sr^-1. Average profiles of R(H) and β^a_π(H) for the main seasons, the annual behaviour of the altitude of the peak scattering ratio and of the tropopause height, and the eigenvectors of the correlation matrix of R(H) are also presented for this period. In succeeding years, the behavior of the stratospheric aerosol layer (SAL) was primarily determined by the Pinatubo volcanic aerosol. Based on an analysis of the scattering ratio profiles, the formation of the vertical volcanic aerosol stratification above Tomsk is demonstrated. The maximum values of Σβ^a_π≈4×10^-3 sr^-1, caused by the volcanic aerosol, were observed in January–February 1992. Between 15 and 30 km, the 1/e-decay time of the stratospheric volcanic aerosol was 357±37 days. Lidar measurements carried out on certain days simultaneously at λ=532, 353, and 628 nm allowed us to investigate the transformation of the exponent A(H) that characterizes the spectral dependence of the volcanic aerosol backscattering coefficient in the wavelength range 353–532 nm and to estimate the geometric cross section of particles S(r).