Atmos. Meas. Tech., 11, 4345–4360, 2018 https://doi.org/10.5194/amt-11-4345-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Exploring femtosecond laser ablation in single-particle aerosol mass spectrometry Ramakrishna Ramisetty1, Ahmed Abdelmonem1, Xiaoli Shen1, Harald Saathoff1, Thomas Leisner1, and Claudia Mohr1,2 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany 2Department of Environmental Science and Analytical Chemistry, Stockholm University, Stockholm, Sweden Correspondence: Claudia Mohr ([email protected]) Received: 29 September 2017 – Discussion started: 10 October 2017 Revised: 11 June 2018 – Accepted: 21 June 2018 – Published: 23 July 2018 Abstract. Size, composition, and mixing state of individual firing fs laser with the single-particle mass spectrometer em- aerosol particles can be analysed in real time using single- ploying the fs laser light scattered by the particle to trigger particle mass spectrometry (SPMS). In SPMS, laser abla- mass spectra acquisition. Generally, mass spectra exhibit an tion is the most widely used method for desorption and ion- increase in ion intensities (factor 1 to 5) with increasing laser ization of particle components, often realizing both in one power density (∼ 109 to ∼ 1013 W cm−2/ from ns to fs laser. single step. Excimer lasers are well suited for this task due At the same time, fs-laser ablation produces spectra with to their relatively high power density (107–1010 W cm−2/ larger ion fragments and ion clusters as well as clusters with in nanosecond (ns) pulses at ultraviolet (UV) wavelengths oxygen, which does not render spectra interpretation more and short triggering times. However, varying particle optical simple compared to ns-laser ablation. The idea that the higher properties and matrix effects make a quantitative interpreta- power density of the fs laser leads to a more complete parti- tion of this analytical approach challenging. In atmospheric cle ablation and ionization could not be substantiated in this SPMS applications, this influences both the mass fraction of study. Quantification of ablated material remains difficult due an individual particle that is ablated, as well as the result- to incomplete ionization of the particle. Furthermore, the fs- ing mass spectral fragmentation pattern of the ablated ma- laser application still suffers from limitations in triggering it terial. The present study explores the use of shorter (fem- in a useful time frame. Further studies are needed to test po- tosecond, fs) laser pulses for atmospheric SPMS. Its objec- tential advantages of fs- over ns-laser ablation in SPMS. tive is to assess whether the higher laser power density of the fs laser leads to a more complete ionization of the en- tire particle and higher ion signal and thus improvement in the quantitative abilities of SPMS. We systematically inves- 1 Introduction tigate the influence of power density and pulse duration on airborne particle (polystyrene latex, SiO2, NH4NO3, NaCl, Atmospheric aerosols are known to have large impacts on and custom-made core-shell particles) ablation and repro- climate change, air quality and human health, and these ef- ducibility of mass spectral signatures. We used a laser abla- fects are strongly related to the chemical composition of in- tion aerosol time-of-flight single-particle mass spectrometer dividual aerosol particles (Fuzzi et al., 2015). Atmospheric (LAAPTOF, AeroMegt GmbH), originally equipped with an aerosols are highly heterogeneous in composition due to the excimer laser (wavelength 193 nm, pulse width 8 ns, pulse vast number of natural and anthropogenic sources, as well as energy 4 mJ), and coupled it to an fs laser (Spectra Physics transformation and mixing processes during their residence Solstice-100F ultrafast laser) with similar pulse energy but time in the atmosphere (Kulkarni et al., 2011). Most analy- longer wavelengths (266 nm with 100 fs and 0.2 mJ, 800 nm ses of aerosol chemical composition focus on the bulk, not with 100 fs and 3.2 mJ). We successfully coupled the free- least due to the very small mass and number of molecules present in an average atmospheric particle, making single- Published by Copernicus Publications on behalf of the European Geosciences Union. 4346 R. Ramisetty et al.: Exploring femtosecond laser ablation particle studies challenging. However, the mixing state and ter optics. The power density is much higher for femtosec- composition of individual particles are crucial pieces of in- ond (fs) laser pulses compared to nanosecond laser pulses. formation for the assessment of particle interaction with light The interaction of a high-intensity laser beam with a solid or water vapour and thus their contribution to climate change particle leads to the generation of a plasma that increases its (Charles, 2012; IPCC, 2007; Jacobson, 2005; John, 2016; energy content, its average charge state, and its charge den- Laskin et al., 2018). sity during the pulse duration. Zhou et al. (2007) used a one- Single-particle mass spectrometry (SPMS) is a powerful dimensional hydrodynamic model and experimental observa- tool for the investigation of the size-resolved chemical com- tions to explain ns-laser pulse interactions with particles and position of individual atmospheric aerosol particles in real observed that the laser energy absorption efficiency and thus time (Brands et al., 2011; Gaie-Levrel et al., 2012; Mur- ionization efficiency increase with shorter pulse width (from phy, 2007; Murphy et al., 2006; Pratt et al., 2009; Pratt and 10 ns to 10 picoseconds, ps). Molecular dynamic model sim- Prather, 2012; Zelenyuk et al., 2010). Although a two-step ulations by Schoolcraft et al. (2001, 2000) explained the pro- approach separating laser ablation and laser ionization bears cess of laser desorption and ionization of submicron particles several advantages for identifying specific molecules (Passig for amorphous and crystalline particles with and without in- et al., 2017) many instruments currently still use single-step clusions as a function of the nature of the material. laser desorption and ionization. Single-particle mass spec- Fs lasers are widely used in the fields of micromachining trometers currently in use by different groups worldwide and nanoparticle ablation (Chichkov et al., 1996; Gattass and have very similar designs (Gaie-Levrel et al., 2012; Johnston, Mazur, 2008; Malvezzi, 2014; Richard et al., 2013; Tsuji et 2000; Murphy and Thomson, 1995; Zelenyuk et al., 2009). al., 2003). Fs-laser ablation mechanisms include Coulomb They usually consist of one or two scattering lasers that de- explosion (soft ablation) and phase explosion followed by tect the particle size by particle time-of-flight, an ionization thermal ablation (strong ablation), depending on fs-laser in- laser, commonly a nanosecond (ns) excimer laser for parti- tensities (Amoruso et al., 1999; Leitz et al., 2011; Roeter- cle desorption and ionization in one single step, and the mass dink et al., 2003; Zhou et al., 2007). Coulomb explosion analyser (Murphy, 2007). and/or phase explosion happen in multi-photon ionization, Quantitative analysis of single aerosol particles via depending on pulse energy. Because of the very short inter- laser ablation remains challenging, although several studies action time in the femtosecond laser ablation and ionization, achieved advancements (Bhave et al., 2001; Fergenson et al., it is considered to be almost instantaneous, with the result- 2001; Gross et al., 2005; Healy et al., 2013), e.g. by de- ing kinetic energy of the electrons high enough to immedi- tailed characterization of instrument sensitivity for individ- ately escape the target. Therefore, no space charge shield- ual chemical species and by optimizing ionization laser pa- ing of the sample should occur. Consequently, the target is rameters to reduce fragmentation. However, so far no single- left behind with a corresponding density of localised pos- particle mass spectrometer is available for quantitative on- itive holes. Once a sufficient density of holes is achieved, line analysis of particle mixtures. Single-step laser desorp- the target surface becomes electrostatically unstable, result- tion and ionization with excimer lasers is highly non-linear ing in a Coulomb explosion of ions, the Coulomb explosion (Zelenyuk and Imre, 2005). Usually, particles are not com- takes place in the initial phase and/or phase explosion occurs pletely ablated (Ge et al., 1996; Murphy, 2007), and the abla- at a higher stage of multi-photon ionization (Roeterdink et tion process leads to irreproducible spectra. The dominant al., 2003). Various mechanisms of the fs-laser ablation (exci- yield of species with low ionization potential further lim- tation, melting, ablation) were compared to the nanosecond its the quantitative ability of ns-laser SPMS (Reilly et al., laser ablation (Harilal et al., 2014; Malvezzi, 2014) at dif- 2000). The absorption of photons depends on the optical ferent timescales. Substantial atomization and strong clus- properties of the chemical components of the particle, with ter formation are the major effects due to the phase and/or important implications for core-shell or multi-component Coulomb explosion in the fs-laser ablation (Malvezzi, 2014; particles (Cahill et al., 2015). Reported approaches to im- Roeterdink et al., 2003; Xu et al., 2000; Zaidi et al., 2010). prove the quantitative abilities of SPMS include two-step The fs-laser ablation generates more atomic ions than in the vaporization–ionization, where a CO2 laser was used prior nanosecond laser ablation due to rapid energy transfer and to excimer laser ionization for the evaporation of the parti- also leads to the formation of more ion clusters because cles (Cabalo et al., 2000; Morrical et al., 1998; Smith et al., of the explosions. A brief comparison between nanosec- 2002; Whiteaker and Prather, 2003; Woods et al., 2002), or ond and femtosecond laser ablation mechanisms for differ- the use of a high power density Nd:YAG laser of 5 ns pulse ent timescales is given in the Supplement (Fig.