Time-of-flight mass spectroscopic detection of new elemental and mixed small atomic clusters in the laser evaporation of carbon nitride Sergei I. Kudryashov,* Byong K. Kim, Jong I. Kim, Nikita B. Zorov and Yurii Ya. Kuzyakov Department of Chemistry, M. V. Lomonosov Moscow State University, 119899 Moscow, Russian Federation. Fax: +7 095 932 8846; e-mail: serge@laser.chem.msu.ru New, positively-charged elemental and mixed atomic carbon–nitrogen clusters have been detected by time-of-flight mass spectroscopy in the laser evaporation of carbon nitride C 3 N 4.25 . Many elemental and mixed atomic clusters of carbon and nitrogen — p romising materials in the production of high-energy density materials — demonstrate relative stability, according to recent ab initio calculations. 1,2 Meanwhile, up to date experimental studies on the structural, physical and chemical properties of these clusters are very rare because production of the clusters by laser vaporisation source techniques is impeded by a lack of nitrogen-rich carbonaceous targets. 3–6 In this case, current mass spectroscopic studies of the gas-phase evaporation products of carbon–ni trogen materials, usually related to the laser deposition of superhard carbon nitride (b-C 3 N 4 ) thin films, 7,8 can also provide a clearer understanding of the nature of nitrogen and nitrogen–ca rbon atomic clusters. In this work positively charged products from the laser evaporation of carbon nitride C 3 N 4.25 (a polymeric material with a symm-heptazine monomer produced by thermal decomposition of mercuric rhodanide 9 ) were studied by time-of-flight mass spectroscopy. The sample was prepared from a purified powder of polymer carbon nitride (PCN) pressed at 50 bar to a pellet (diameter 0.7 cm, thickness 0.4 cm, bulk density 1.6 g cm –3 ). Time-of-flight studies were carried out using a commercial quadrupole MX-7304 mass spectrometer modified for time-of- flight mass analysis (Figure 1). Operation conditions for the linear mass spectrometer were maintained by pumping the vacuum chamber with a high-vacuum discharge pump up to 10 –7 Torr. The pulsed second harmonic output of a Q-switched Nd:YAG laser [laser wavelength l = 532 nm, pulse energy E = 4.5±0.3 mJ, pulse duration (FWHM) t = 10±1 ns, pulse repetition rate f = =0.92±0.05 Hz] attenuated by neutral calibrated filters and a LiNbO 3 polariser was focused by a lens (F = 28 cm) onto the surface of a PCN target at a slight angle (10–15°). A small part (8%) of the laser radiation energy was directed by a beam splitter onto a photodiode and a pyroelectric plate to synchronise the detection system and to control the laser energy per pulse. Primary positive ions generated from the PCN target were extracted and accelerated by a pulsed voltage (–100V, pulse width 0.1–10 ms) applied to a grid placed at a distance of 4 cm in front of the target with a ground graphite ring supporting the target. The cluster ion beam, with an adjustable mechanical momentum of the ions, was mass-analysed during drift in a field-free region (45 cm) to the input of a secondary electron multiplier with a time constant of 3 ns. After discrimination of noise and pre-amplification separate bursts of positive pulses of amplitude +4 V corresponding to individual cluster ions were digitised for 320 ms by a pulse counter connected to a PC via an interface. The digital signal was averaged over 100 laser shots and was then recorded. The time-of-flight spectra of positive cluster ions were obtained during laser evaporation of the PCN target with laser power density 0.3 GW cm –2 . The calibration curve for the cluster ion mass versus flight time obtained for the cluster source conditions allowed the determination of ion mass values to an accuracy of 0.1% and reproducibility 0.3%. Maximum initial kinetic energies of ions measured by retarding potential technique were equal to 0.5 << eU » 100 eV. Corresponding temperature of the cluster ion source T < 6000 K was favourable for preferable formation of singly charged ions. Cluster ion masses were calculated using equation (1) for the acceleration and drift of singly charged ions: where M is the cluster ion mass (a.m.u.) related to its charge (Z = 1), e is the charge of an electron (C), U is the accelerating voltage (V), l is the distance from the target to the accelerating grid (m), L is the drift tube length (m), m 0 is one twelfth part of the carbon atom mass (carbon unit of mass, kg), t is the flight time (s) and t is the accelerating electric pulse width (s). According to the formula the maximum mass of a cluster ion detected with the mass analyser is proportional to the accelerating pulse width and increases from 200 to 1700 a.m.u., with growth of the latter in the range 1–1 0 ms. Pulsed acceleration of cluster ions to equal momentum is favourable in the resolution of high molecular mass ions due to the linear dependence of ion mass on flight time, as compared with monoenergetic ion beams which are described by a square root dependence. In this case, mass resolution of the cluster ion beam over the whole mass range was independent of cluster ion mass and was limited only by time resolution of the detection system t c = 1.25±0.05 ms (counting cycle of the pulse counter). Then mass resolution of the nearest peaks in the mass spectra of positive clusters with a difference in mass of D a.m.u. was provided by their detection within the next counting cycle: Taking into account equation (2), the numerical expression for mass resolution of the cluster ion beam is of the form D (a.m.u.) = 1.2t (ms) which corresponds to resolution of the two nearest peaks for mixed carbon–nitrogen cluster ions (with D = 2 a.m.u.) by using an accelerating electric pulse of width t <2 ms. The abundances of the cluster ions detected were multiplied by coefficient M 0.5 , thus accounting for the dependence E/M 0.5 of the ion-electron conversion efficiency of a secondary electron 1 2 3 4 5 6 7 8 9 10 11 12 11 Figure 1 Scheme for the linear time-of-flight mass spectrometer: 1 – laser, 2 – beam splitter, 3 –m irror, 4 –fo cusing lens, 5 – vacuum chamber, 6 – polymer carbon nitride target supported on a ground graphite ring, 7 – extracting/accelerating grid and ion drift tube of mass analyser, 8 – secondary electron multiplier, noise discriminator and pre-amplifier, 9 – photodiode and pyroelectric plate, 10 – computer with pulse counter and interface, 11 – pr evacuum pump and high-vacuum discharge pump, 12 – thermocouple and ionisation gauges. M = (1) eUt(t – 0.5t) l(L + l)m 0 t(N + D)– t(N)= D ³ 2t c Llm 0 eUt (2) Mendeleev Commun., 1999, 9(1), 1–1 – 1 –