Time-of-flight mass spectroscopy of femtosecond and nanosecond laser ablated TeO2 crystals

International Journal of Mass Spectrometry 299 (2011) 5–8 Contents lists available at ScienceDirect International Journal of Mass Spectrometry journa l homepage: www.e lsev ier .co Time-o nd laser ab S. Bekea, nse a Laser Technol -0198 b BAM Federal ny a r t i c l Article history: Received 10 Ju Received in re Accepted 24 A Available onlin Keywords: Time-of-flight Tellurium diox Femtosecond l Nanosecond la Ablation durat ser ab yze t the tensit d spe e form eO+ f serve ener ion. 1. Introduction Tellurium dioxide crystal (TeO2) is a transparent insulator, which is us modulators optical hom optical pow TeO2 hashig part of the interest for femtosecon microproce dynamics a there are s standing of time-of-flig intensity ra plasma pre has been w rials [6–12] Here we of our refle produced b ∗ Correspon E-mail add ilar laser wavelengths and focusing conditions throughout a wide range of mass-to-charge ratios. This process provides a universal detection method of ions over a wide mass range. In this paper, we 1387-3806/$ – doi:10.1016/j. ed in acousto-optical devices (beam deflectors, light , tunable optical filters). This material features high ogeneity, low optical absorption and scattering, high er capability and high optical damage resistance [1,2]. h refractive indices and it transmits in themid-infrared electromagnetic spectrum; hence, it is of technological optical devices. Our previous studies have revealed that d (fs) laser ablation has a great potential for the surface ssing of single-crystalline (c-TeO2) TeO2 [3,4]. In the nd mechanism of fs-laser ablation of TeO2, however, till many open questions. For a more detailed under- the mechanism, an in situ study is indispensable. Using ht mass spectrometry (TOFMS) one can analyze the tios of different ionic species generated in the ablated cisely, reproducibly and accurately [5]. In fact, TOFMS idely applied for laser ablation studies of various mate- so far, but never for TeO2 to the best of our knowledge. take benefit of the excellent (single-pulse) sensitivity ctron TOFMS setup in order to study the ionic species y the laser ablation with fs- and ns-laser pulses of sim- ding author. resses: [email protected], [email protected] (S. Beke). use this system to investigate in situ the fs-laser ablation of TeO2 in comparison with ns-laser ablation. 2. Experimental 2.1. Crystal growth procedure and sample preparation Single-crystals of paratellurite (TeO2) were grown by using the balance controlled Czochralski method. The description of the growth apparatus is given in Ref. [13]. For the crystal growth, a resistance furnace was employed in order to maintain low thermal gradients and minimize the eventual temperature fluctuation dur- ing thegrowthandpost-growthprocesses. The crystalsweregrown from 6N pure TeO2. Single-crystals were pulled along the 〈110〉 direction in sizes up to 50mmindiameter and50mmin length. The seed rotation rate varied between 15 and 20 rounds perminute and the pulling rate was 0.8–1mm/h. For the measurements samples were cut with a diamond saw and X-ray oriented with a precision better than 0.5◦. Polished (110) TeO2 surfaces were prepared by a standard method using SiC for grinding and AB Alpha Alumina (Buehler Linde A 0.3�m) for mechanical polishing [3,13]. Paratel- lurite crystals have a band gap energy of 4.05 eV. Hence, we assume 2-photon absorption processes (as the single photon energies are 3.1 eV and 3.5 eV for the two relevant wavelengths of 398nm and see front matter © 2010 Elsevier B.V. All rights reserved. ijms.2010.08.022 f-flight mass spectroscopy of femtoseco lated TeO2 crystals ∗, T. Kobayashia, K. Sugiokaa, K. Midorikawaa, J. Bo ogy Laboratory, RIKEN – Advanced Science Institute, Hirosawa 2-1, Wako, Saitama 351 Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germa e i n f o ne 2010 vised form 24 August 2010 ugust 2010 e 6 September 2010 mass spectroscopy ide crystals aser ser a b s t r a c t Single-pulse femtosecond (fs) (pulse duration 4ns, wavelength 355nm) la mass spectrometer (TOFMS) to anal crystalline telluria (c-TeO2, grown by three-order difference of the peak in observed regarding the laser-induce charged Te ions (Te+, Te2+, Te3+) in th In case of the ns-laser ablation the T was no Te trimer (Te3+) formation ob depended on the applied laser pulse byproduct in both kinds of laser ablat m/locate / i jms and nanosecond b , Japan ion ∼200 fs, wavelength 398nm) and nanosecond (ns) (pulse lation have been applied in combination with time-of-flight he elemental composition of the plasma plume of single- balance controlled Czochralski growth method). Due to the ies of the ns and fs-laser pulses, significant differences were cies in the plasma plume. Positive singly, doubly and triply of their isotopes were observed in case of both irradiations. ormation was negligible compared to the fs case and there d. It was found that the amplitude of Te ion signals strongly gy. Singly charged oxygen ions (O+) are always present as a © 2010 Elsevier B.V. All rights reserved. 6 S. Beke et al. / International Journal of Mass Spectrometry 299 (2011) 5–8 Fig. 1 355nmuse cause local energy leve result in ele note, that a prior to the types of cha TeO2) such hole center levels in the 2.2. Time-o Fig. 1 sh the TOFMS Femtosecon system (Hu rate. Thewa duration. T thick secon a 398nm fe filter (03FC length) was NIR fs-laser tion experim The typi band-pass through att view port o chanical sh the beam p pulses wer length onto laser spot d in the short sample was by 150�m pulse ablat 10−7 Torr. The TOF positive ion tion normal to the samp distance of from the ce collimator w reflected fro (elevation angle � =3.5◦). The termination voltage was +6kV. Those ions were temporally separated by their mass-to-charge ratios during their flights and were detected using a three-stage MCP. tal flight path length was 1.7m. Output signals from the ere r Cor F me by optic divi erag or. ome ults ch “T 55n h alm a st lter. samp aser antl nt sp ce el acq peri nduc tion lly re m re y tra er, i exp tify ted d with accel can b the ality ent tion, ratio OFM ults mtos . Schematic of the TOFMS set up for the fs and ns ablation. d in this study). In fact, impurities andcrystal defects can changes in the band structure with additional allowed ls in the band gap. For instance, oxygen vacancies may ctron-donor states below the valence band [14]. Please blative removal of the thin gold layer from the surface experiments (see Section 2.2) could also cause various nges in the TeO2 crystal (by unintentional irradiation of as charged oxygen vacancies and non-bridging oxygen s, which contribute to the appearance of new energy forbidden gap [15]. f-flight analysis and laser processing of c-TeO2 ows the schematic of the experimental setup used for experiments allowing the use of different laser sources. d laser pulses were supplied from a Ti:Sapphire laser rricane; Spectra-Physics) operating at 500Hz repetition velengthof the laserpulsewas796nmwith120 fspulse he laser pulses were frequency-doubled using a 1mm dharmonic generation crystal (BiBO crystal) resulting in mtosecond laser pulse of ∼200 fs duration. A band-pass G033; Melles Griot, transparent to the 398nm wave- placed to suppress the non-converted fraction of the pulses. The filter was used only in the fs-laser irradia- ents. cal fs pulse energy of 115�J after passing through the filter was adjusted in the 0.1–7.3�J range by passing enuating neutral density (ND) filters placed in front of a f the TOF vacuum chamber. A fast-response electrome- utter (Uniblitz LS6; Vincent Associates) was placed in ath to perform single-pulse measurements. The laser e then focused by an achromatic lens of 70mm focal The to MCP w LeCroy The TO diation of the each in cent av detect In s the res Resear laser: 3 throug placing pass fi on the of ns l signific differe Sin proper tion ex thin co irradia to loca This fil ment b Howev flight. The to iden genera TOFMS being (TOF) q being portion instrum calibra charge of theT ∼300. 3. Res 3.1. Fe the sample surface at an incident angle of 45◦. The iameter (1/e2) was approx. 20�m at the sample surface direction of the elliptical spot. The 2mm thick c-TeO2 mounted on an XY-translation stage and was moved after each irradiation (data acquisition) event. The laser ion was carried out under high vacuum conditions of MS was operated in a reflectron mode. In this mode, s are accelerated by an external electric field in a direc- to the sample substrate. Themaximumvoltage applied le mount was +5kV. A cathode mesh was placed at a 6mm from the sample and kept at 0V. Ions traveling nter of the flight path were blocked by an aluminum ith a hole (6mm diameter). The accelerated ions were m a potential gradient formed by reflection electrodes Fig. 2 sh TeO2 by the ence 1.0 J cm visibility of temporal ra between2 a as H+, C+ a 15.1�s and respectivel tified as Te respectivel heaviest sp Te2O+, Te2O (Te3+) betw ecorded with a digital oscilloscope (WaveRunner 6050; p.) through a fast preamplifier (model 9305; Ortec). asurements were synchronized to the laser pulse irra- means of a photodiode signal triggered from a part al pulse reflected at the band-pass filter. The data of dually recorded spectrum were smoothened by adja- ingover 20datapoints to reduce thenoise fromtheMCP complementary experiments, in order to compare with obtained by the fs laser, a nanosecond laser (New Wave empest”, ThirdHarmonicGeneration (THG)of aNd:YAG m wavelength, 4ns pulse duration) beam was passing ost the same beam path as the fs-laser pulses just by eering mirror into the setup and by replacing the band- Care was taken to realize a similar laser spot diameter le surface as in the fs-case. In this case, the wavelength is close to that of fs laser, while the peak intensity is y different by the order of three, which should produce ecific species in the ablation plume. ectrical charging of the surface critically affects the uisition of TOFMS spectra, prior to the laser irradia- ments the c-TeO2 samples were sputter-coated by a tive layer of gold (∼20nm thickness). Therefore, a pre- sequence of 5–10 laser pulses per spot was required move the gold film and prepare a clean TeO2 surface. moval was monitored in situ during the TOFMS experi- cking the Au+ peak until it vanished almost completely. n some spectra it still can be noticed at 32�s time of erimentally measured times of flight have been used the different positively charged ionic species which are uring laser ablation and which are passing through the out recombination. Based on the energy balance of ions erated in an external electric field, the time of flight e calculated according to TOF = k √ m/q, with m and mass and the charge of the ions, respectively. The pro- constant k includes experimental factors related to the settings and characteristics. Once k is known from a the latter equation can be used to extract the mass-to- directly from the TOF. The maximum mass resolution Sunder thegivenconditionswasestimated tobem/�m and discussion econd laser ablation ows typical TOFMS spectrum, recorded upon ablation of single fs-laser pulse at a laser pulse energyof 3.0�J (flu- −2, peak intensity 6.4×1012 Wcm−2). For the better the peaks, the spectrum is divided into three different nges (a: 0–20�s, b: 20–30�s, c: 30–45�s). The peaks nd10�s [Fig. 2(a)] are attributed to light elements such nd O+ arriving the earliest at the detector. At 13.1�s, 17�s, the O2+, Te3+ and TeO23+ peaks are observed, y. Thepeaks ranging from20 to30�s [Fig. 2(b)] are iden- +, TeO+ and TeO2+ ions at 25.6�s, 27.2�s and 29�s, y. Fig. 2(c) shows the 30–45�s domain in which the ecies appear such as the singly charged Te dimer (Te2+), 2 +, Te2O3+, Te2O4+ and the singly charged Te trimer een 36 and 45�s. S. Beke et al. / International Journal of Mass Spectrometry 299 (2011) 5–8 7 Fig. 2. Typical TOFMS spectra of the TeO2 using a single-pulse fs irradiation in the range: (a) 1–20�s, (b) 20–30�s and (c) 30–45�s. The pulse energy of the femtosec- ond laser was 3.0�J/pulse (fluence 1.0 J cm−2, peak intensity 6.4×1012 Wcm−2). Fig. 3 presents a close up of the Te monomer and dimer peaks when using 2.1�J/pulse energy (fluence 0.7 J cm−2, peak inten- sity 4.5×1012 Wcm−2). The high-resolution view of Te+ peak ion reveals that 6 different isotopes (122Te, 124Te, 125Te, 126Te, 128Te, 130Te) are clearly present in the spectrum (Fig. 3(b)). Tellurium has eight natural isotopes of which six are recognizable in our spec- trum. The other two peaks are not visible due to the low relative quantity ra tions of the different Te as shown in 3.1.1. Influe during fs-las Increasin tion of new Fig. 4. The la first ion sig peak intens The first Te old pulse e Fig. 3. Close u cating the pres fluence 0.7 J cm OFMS of c-TeO2 using a single-pulse fs irradiation in the TOFMS range . The pulse energy was changed from 0.7 to 7.3�J/pulse. �s and it is associated with the detection of Te+ [Fig. 4(a)]. ak is broadened due to the presence of the different Te iso- as discussed in Section 3.1). From 1.2�J laser pulse energy e 0.4 J cm−2, peak intensity 2.6×1012 Wcm−2), the TeO+ ogether with the Te dimer (Te2+) peak start to evolve at �s and ∼36.5�s [Fig. 4(b)–(g)]. The O+ peak at 9.4�s is t throughout the whole energy range indicating that singly oxygen is alwaysproduced in the courseof fs-laser ablation. 3�J laser pulse energy (fluence 1.0 J cm−2, peak intensity 012 Wcm−2), the Te2O2+ and the Te2O3+ peaks appear at �s and 39.8�s, respectively [Fig. 4(e)]. Te trimer (Te3+) at tio. The close up of the Te2+ peak shows the combina- isotopes. Taking into account the probability of the isotopes (taken from Ref. [16]), the peaks are assigned Fig. 3(c). nce of the fs pulse energy on the species produced er ablation g the fs-laser pulse energy can give rise to the forma- peaks and species in the spectrum as demonstrated in ser pulse energywas systematically increased until the nals appeared. Below 0.7�J/pulse (fluence 0.2 J cm−2, ity 1.5×1012 Wcm−2) no relevant signal was detected. O2 ablation related ion signals appear at a thresh- nergy of 0.7�J/pulse. The signal can be observed at Fig. 4. T 1–45�s ∼25.6 The pe topes ( (fluenc peak t ∼27.1 presen ionized At 6.4×1 ∼38.9 p of the Te monomer and dimer peaks in the TOFMS spectrum indi- ence of several isotopes in the ablation plume (fs pulse energy 2.1�J, −2, peak intensity 4.5×1012 Wcm−2). 44.7�s bec seen in Fig. peak slight starts to de favor the Te pulse energ Above 3 TeO23+) are peak intens observed in integratem due tomuc gies were u S+ and K+ io the surface shown in F TeO2 to avo omes visible [a higher magnification of this peak can be 2(c)]. Up to 3�J/pulse, the amplitude of the Te dimer ly increases, however at higher energies (above 3�J) it crease indicating that high laser pulse energies do not dimer formation. TeO+ peak also decreaseswith higher y. �J pulse energy double and triple charged species (Te2+, also observed, which can be explained by the high ity. In case of fs-laser irradiation light elements are also the spectra (in the 1–15�s range) since fs pulse candis- aterials into atomsmore efficiently than ns-laser pulses h higher peak intensity [17].When low laser pulse ener- sed (0.7–1.7�J), several different peaks of Na+, Al+, Si+, ns can be observed originating from contaminations on . The Gold ions (197Au+) can be recognized at 32�s as ig. 4(a) deriving from the gold thin films deposited on id the electronic charging. 8 S. Beke et al. / International Journal of Mass Spectrometry 299 (2011) 5–8 Fig. 5. TOFMS 1–45�s. The p Te+ is pr its flight (m position wh observed at ated with th high laser e resolution b The distorti due to the not appear quickly befo 3.2. Nanose In case in case of th energy. The observed at intensity 3. there is alm are observe – in contra peak intens the formati Oxygen energies e 6.4×109 W the ns abla Te dimer ( pulse energ 43 J cm−2, p ing that hig assumed he dimer ions and only singly charged O+ and Te+ are produced as it was also observed in case of the fs irradiation. The shoulder at around 24.5�s is visible also in case of the ns ablation when the pulse energy is higher than 105�J (fluence 34.2 J cm−2, peak intensity 8.4×109 Wcm−2), indicating that its origin does not depend on the laser pulse duration and the peak intensity, however it happens in both cases when using large pulse energy. clus his st ) an nof s . to th laser er-in ions ablat of t red t ed. Te io plied ion o he en ed w det lso c atom peak wled eke i Prom nces chida of c-TeO2 using a single-pulse ns irradiation in the TOFMS range of ulse energy was changed from 43 to 132�J/pulse. oduced by the dissociation of the Te dimer (Te2+) during etastable dissociation). The shifted time depends on the ere metastable dissociation takes place. The shoulder around 24.5�s in case of high pulse energies is associ- e distortion of the Te+ peaks. This happens often when nergy is used. Along with this phenomenon, the mass ecomes worse as it can be observed in Fig. 4(e) and (f). on is caused by the shielding of the acceleration voltage high-density plasma induced by laser ablation. It does for light ions such as H+ and C+ since they travel away re the high-density part of the plasma cloud develops. 4. Con In t 398nm ablatio TOFMS Due and fs- the las tive Te in the In case compa observ The the ap format when t observ usually study a als into higher Ackno S. B for the Refere [1] N. U cond laser ablation of ns-laser irradiation, the same method was used as e fs irradiation to determine the threshold laser pulse results are shown in Fig. 5. The first ion signals were a laser pulse energy of 43�J (fluence 14 J cm−2, peak 4×109 Wcm−2) [Fig. 5(a)]. It is interesting to note that ost no formation of TeO+ and multiple charged species d in the course of ns ablation even at large pulse energy st to the fs ablation. The three-order difference in the ity between the fs and ns-laser pulses seems crucial in on of these species. ions 16O+ (9.3�s) are produced for laser pulse xceeding 80�J (fluence 26.1 J cm−2, peak intensity cm−2) [see Fig. 5(c)–(e)]. The main species observed in tion are Te+ and O+. The signal of the singly charged Te2+) (37.2�s) gradually decreases with increasing y. It disappears almost completely at 132�J (fluence eak intensity 10.5×109 Wcm−2) [Fig. 5(e)] indicat- h laser energy restrains the Te dimer formation. It is re that high laser fluence disables the formation of Te [2] Ochmach [3] S. Beke, K Phys. 43 [4] S. Beke, K [5] F. Poitras 6184. [6] Y.Matsuo Y. Hayash [7] R. Stoian Surf. Sci. [8] H. Varel, Sci. 127– [9] T. Kobaya Appl. Phy [10] T. Kobaya Surf. Sci. [11] T. Kobaya J. Chem. P [12] T. Kobaya J. Chem. P [13] F. Schmid [14] Anderson [15] Guangmi [16] Chronolo vatory, M [17] M. Kurata K. Midori ions udy single-pulse fs (pulse duration ∼200 fs, wavelength d ns (pulse duration 4ns, wavelength 355nm) laser ingle-crystallineTeO2 wascarriedoutusinga reflectron e three-order differenceof thepeak intensities of thens pulses, significant differenceswere observed regarding duced species in theplasmaplume. Singly chargedposi- in the formof six isotopesweredetectedpredominantly ion plume in case of both ns and fs-laser irradiations. he ns-laser ablation the TeO+ formation was negligible o the fs case and therewasnoTe trimer (Te3+) formation n peak intensities in the TOFMS strongly depended on laser pulse energy. Low laser pulse energies favor the f Te dimer species, however continuously decreased ergywas increased. Tellurium trimer specieswere also hen femtosecond laser (3�J/pulse) was used. O+ was ected as a byproduct of the ns and fs-laser ablation. 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Time-of-flight mass spectroscopy of femtosecond and nanosecond laser ablated TeO2 crystals Introduction Experimental Crystal growth procedure and sample preparation Time-of-flight analysis and laser processing of c-TeO2 Results and discussion Femtosecond laser ablation Influence of the fs pulse energy on the species produced during fs-laser ablation Nanosecond laser ablation Conclusions Acknowledgement References