it can be generated in 
 various ways , for example , when a gas is heated to a very high temperature , 
 or when an electric discharge passes through a gas or in the plume 
 of vapor produced when a high - powered laser impinges on a solid . the 
 composition of a plasma depends on its effective temperature . at the 
 highest temperatures ( as exist in the core of stars ) , it consists 
 of fundamental particles and small monatomic ions . in regions cooler 
 than found within stars , such as circumstellar shells and the hotter parts of the interstellar medium , electrons 
 and molecular - ion fragments can be present and provide the opportunity 
 for molecule building on dust grains . it is well - known that electric discharges in gases can produce 
 small , highly stable molecules and also 
 hitherto unobserved species . laser ablation 
 of solids is another convenient laboratory method of producing plasmas 
 and has been employed in combination with microwave spectroscopy for 
 a variety of purposes , including vaporization of refractory solids , the production and characterization of free radicals , and the synthesis of new species . a nd : yag laser operating on its second harmonic at 532 nm , with 
 a pulse duration of ca . 10 ns and an energy of 10 mj / pulse 
 is sufficient to ablate metals ( such as copper , silver , gold , and 
 many others ) to produce a small plume of plasma composed of metals 
 ions , atoms , and electrons . if this plume immediately interacts with 
 a gas of neutral molecules in its vicinity , fragmentation of the molecules 
 can occur and the fragments can then undergo reactions . for example , 
 interaction of a sf6/ar gas mixture with a plume of laser - ablated 
 copper gives cuf and arcuf , both of which were detected by their rotational 
 spectra . sometimes small , thermodynamically stable , well - known species result . 
 when a nh3/ ccl4/ar gas mixture meets a plume 
 of ablated copper , one product is cyanogen chloride clcn ( unpublished 
 results ) . another product is h3n  cu  cl ( unpublished 
 results ) , previously unknown in the gas phase . broadband rotational 
 spectroscopy provides a powerful new method by which to analyze gas - phase molecules generated within plasma 
 and an opportunity to examine some exotic chemistry . in this article , 
 we report the synthesis and identification by this means of two examples 
 of a simple , interesting molecular type , namely m  cc  cl 
 ( m = ag or cu ) . the rotational spectra were observed by using the 
 upgraded chirped - pulse , 
 fourier transform microwave ( cp - ftmw ) spectrometer at newcastle university . the spectrometer was configured 
 in the 718.5 ghz region with a pulsed nozzle perpendicular 
 to the axis of microwave propagation . at a gas pulse repetition rate 
 of 1.05 hz , 8 free induction decay ( fid ) emissions per gas pulse were 
 detected , each with 20 s duration , and then digitized on an 
 8-bit 25 gs / s oscilloscope after heterodyning . for observing ag  cc  cl 
 and cu  cc cl , a pulse of a gas mixture containing 
 roughly 1% ccl4 balanced in 6 bar of argon , was flowed 
 over either a silver or copper target rod which was ablated with a 
 nd : yag laser ( continuum minilite , 532 nm , 10 mj / pulse of 10 
 ns duration , repetition rate is once per nozzle pulse ) and subsequently 
 expanded into the vacuum chamber . the final data sets represent a 
 time - domain average of 2 m fids for ag  cc  cl 
 and 3 m fids for cu  cc cl , which were subsequently 
 fourier transformed with an applied high - resolution window function 
 to improve resolving power . for ag cl based isotopologues , an equivalent gas mixture was prepared 
 with ccl4 ( cambridge isotopes ) . for the ag  cc cl sample , a total of 660 
 k fids were collected , while for cu cc  cl the total was 2.2 m. both were fourier transformed 
 in identical fashion . cl 
 ( m = ag or cu ) was carried out by using quantum chemical calculations 
 at the ccsd(t)/aug - cc - pv5z level of theory with the molpro package . for ag , the aug - cc - pv5z - pp basis was employed , 
 where pp indicates that a pseudopotential was used for the core electrons . all electrons were included in the correlation treatment for cu , cl , 
 and c , however , using the basis functions aug - cc - pv5z , aug - cc - pv(5+d ) , 
 and aug - cc - pv5z , respectively we have recently 
 been using the laser - ablation method to make a systematic investigation 
 of the interactions of a range of simple lewis bases b with either 
 agx or cux , where x is a halogen atom . in these experiments , in the case x = cl , a pulse of argon containing 
 approximately 1% of ccl4 is expanded from a pressure of 
 ca . 6 bar through a small hole into a vacuum . before issuing into 
 the vacuum chamber , the gas pulse passes over a silver rod . a suitably 
 timed pulse from a nd : yag laser operating at 532 nm impinges on the 
 metal rod to produce a plasma composed of metal ions , electrons , and 
 ( presumably ) various fragments of the molecules contained in the gas 
 pulse . figure 1 shows a photograph of 
 the plasma plume produced with a silver rod . when the rotational spectra 
 of substances in the gas pulse are detected ( sufficiently far downstream 
 from the point of ablation for the plasma to have decayed ) , it is 
 possible to identify molecules such as agcl and bagcl . photograph 
 of the plasma plume generated by the focused laser pulse 
 above the surface of the silver metal target , taken through a viewport 
 centered on the axis of the expanding gas pulse ( see experimental and theoretical methods for description of spectrometer ) . the plume of vaporized material can be seen propagating from left 
 to right at the center of the image . recorded through a dby39 filter 
 ( optical density of + 7 at 532 nm ) to protect photographic equipment 
 from intense reflected laser light . while attempting to observe the rotational spectrum of n2ag cl 
 by using 
 a mixture containing n2 , ccl4 , 
 and ar in appropriate ratio , we detected a weak , simple spectrum composed 
 of equally spaced groups of transitions ( spacing  1500 mhz ) , 
 each group apparently consisting of a doublet of spacing  
 50 mhz , but on closer examination each component of the doublet itself 
 proved to be a doublet with individual components separated by only 
 a few tens of khz , as illustrated in figure 2 ( measured frequencies are available in the supporting 
 information ) . upper panel shows ( upward pointing ) the rotational spectrum 
 observed 
 ( 2 m fids ) when pulses of supersonically expanded ar / ccl4 gas mixture interact with plasma produced by ablation of a silver 
 rod with 532 nm radiation from a nd : yag laser . the downward - pointing 
 spectrum is that simulated by using the spectroscopic constants given 
 in table 1 . cl are in blue , while transitions of ag  cc  cl and ag  cc  cl are in magenta and green , respectively . measured transitions that do not assign to these species are attributable 
 to other species present within the probed sample , including the diatomic 
 molecule , agcl . the left - hand lower panel shows an expansion of the 
 doublet arising from cl hyperfine structure in the j = 9  8 transition of ag the corresponding 
 doublet ( obtained from 0.66 m fids ) in the j = 11 
  10 transition of ag  cc neither the spacing between the 
 groups nor the splitting internal 
 to each doublet is consistent with n2ag  cl as the source of the observed spectrum , which is ca . 1000 
 times weaker than that observed for the agcl precursor . even when all nitrogen was removed from the gas mixture , the new 
 spectrum persisted . replacement of the ag rod by one of cu caused 
 the observed spectrum to disappear , as did removal of ccl4 from the gas mixture , thereby suggesting that the observed spectrum 
 is associated with a molecule containing only ag , c , and cl atoms . the possibility that the spectral carrier is a complex involving argon 
 with a ( ag , c , cl ) containing molecule will be ruled out later . the nearly equal spacing of consecutive groups of transitions indicates 
 that the source of the observed spectrum is a linear molecule or a 
 symmetric - top molecule or a nearly prolate asymmetric - top molecule 
 of effective rotational constant b  750 mhz . 
 in the frequency range of our spectrometer ( 718.5 ghz ) , as indicated earlier , each j + 1  j transition consists of 
 a major doublet of spacing  50 mhz , with each component itself 
 consisting of a minor doublet of separation only a few tens of khz 
 ( figure 2 ) . the major doublet spacing increases 
 linearly with j and is assumed to arise from two 
 isotopologues associated with the two naturally occurring isotopes 
 of silver , ag and ag ( abundances 51.8% 
 and 48.2% , respectively ) . the observation of only two ag isotopologues 
 rules out a molecule having two or more ag atoms , as does a rotational 
 constant b  750 mhz . the minor doublet spacing , 
 on the other hand , decreases with j. such behavior 
 is characteristic of nuclear quadrupole hyperfine structure . what 
 is the origin of this hyperfine structure ? of the three types 
 of atoms , ag , c , and cl , we can assume that 
 only the most abundant isotopes ag , ag , c ( abundance 98.9% ) and cl ( abundance 75.8% ) 
 occur in the isotopologue responsible for the initially observed spectrum . 
 of these , only cl ( i = 3/2 ) has a nonzero 
 nuclear electric quadrupole moment and can lead to the hyperfine structure . 
 for the range of j observed and at our spectrometer 
 resolution , hyperfine structure associated with a single cl nucleus the presence of two cl atoms would lead to a more complicated 
 hyperfine structure , and this possibility can be excluded . when the 
 observed cl hyperfine doublet frequencies of several j + 1  j transitions of either 
 the ag or the ag isotopologue are fitted 
 by using the familiar hamiltonian for a linear molecule , the resulting cl nuclear quadrupole coupling constant is aa(cl )  77 mhz in each 
 case . a survey of cl nuclear quadrupole coupling constants in the 
 literature reveals that aa(cl ) values of the sign and magnitude observed here are commonly 
 found in molecules in which a single terminal cl atom is attached 
 to carbon , but only if the c  cl 
 bond either coincides with the principal inertia axis a of the molecule or makes a very small angle  with it . when the 
 angle  is not zero , aa(cl ) is given by the usual projection formula aa(cl ) = ( 1/2)0(cl ) ( 3 cos   1 ) , where 0(cl ) is the value of the coupling constant along 
 c  cl bond direction . if  is not very small , a significant 
 reduction in the magnitude of aa(cl ) results . c  cl would have a large angle  , therefore a aa(cl ) much reduced in magnitude 
 and consequently can be ruled out as the spectral source . a weakly 
 bound complex involving ar and a molecule containing ag , c , and cl 
 atoms is unlikely to be a linear or a nearly linear molecule . a weak 
 and long ar to ( ag , c , cl molecule ) intermolecular bond would lead 
 to smaller rotational constants than observed and a cl nuclear quadrupole 
 coupling constant much reduced in magnitude because of a projection 
 of the above type and the effects of large amplitude zero - point averaging . the only simple molecule containing ag , c , and cl 
 consistent with all of the arguments rehearsed above is the linear 
 species ag  cc  cl . a fit of observed frequencies using the program pgopher with a hamiltonian appropriate to the ground 
 state of a semirigid , linear molecule , including cl nuclear quadrupole 
 coupling , leads to the spectroscopic constants b0 , dj , and aa(cl ) given in table 1 . the details of the 
 fit are in the supporting information . 
 if the observed rotational spectrum arises from ag cl , the corresponding 
 spectra of the isotopologues ag  cc  cl and ag  cc cl should be detectable with the same sample , given the natural abundance 
 ( 24.2% ) of cl ( i = 3/2 ) . spectra of 
 both isotopologues were indeed observed with the predicted cl nuclear 
 quadrupole hyperfine structure . the fitted spectroscopic constants 
 for both are included in table 1 , while details 
 of the fits are available as supporting information . cc  cl . by using a 
 sample of ccl4 ( cambridge isotopes , 
 99 atoms % c ) in place of isotopically normal ccl4 , the rotational spectra of ag cl were also observed ( see figure 2 ) and analyzed 
 to give spectroscopic constants ( also given in table 1 ) . note that the ratio aa(cl)/aa(cl ) agrees with the ratio q(cl)/q(cl ) = 1.2688 of the chlorine nuclear electric 
 quadrupole moments , within the quite 
 large experimental errors of the former , as required , and that aa(cl ) is unchanged within the 
 experimental error on substitution of c at both c atoms . the changes in the rotational constant b0 accompanying the various isotopic substitutions are 
 also consistent with bond lengths predicted for the linear molecule 
 ag  cc  cl ( see section 3.4 ) . if the conclusions from the 
 two preceding sections are valid , it should also be possible to generate 
 cu  cc cl but with a copper 
 rod substituted for the silver rod . the rotational spectra of the 
 three isotopologues cu  cc  cl , cu  cc the spectra of all three isotopologues were weaker than 
 those of the ag analogues , as can be seen by comparing the ag in fact , it is clear from examining figure 3 that the transitions displayed for cu , only two different j + 1  j transitions were detected and it 
 was necessary to assume dj unchanged from the parent isotopologue in order to obtain both the cu and cl nuclear quadrupole coupling constants . 
 consequently , the coupling constants have relatively large errors . 
 the rotational spectrum of the isotopologue cu  cc cl transitions arises in part from 
 the additional nuclear quadrupole hyperfine structure that results 
 because both cu ( abundance 69.2% ) and cu 
 ( abundance 30.8% ) have nuclei of spin i = 3/2 and 
 significant electric quadrupole moments . nevertheless , these more 
 complicated spectra could be fitted by using pgopher , with the choice of a standard hamiltonian appropriate to 
 the ground state of a semirigid linear molecule containing two quadrupolar 
 nuclei to give the spectroscopic constants b0 , dj , aa(cl ) , aa(cl ) , aa(cu ) and aa(cu ) included in table 1 . detailed fits are 
 available in the supporting information . observed and the  noise  in the simulations is in fact 
 unresolved hyperfine structure arising from cu and cl nuclear quadrupole 
 coupling . the observed transitions resulted from the collection of 
 3 m fids in the left - hand and center panels and 2.2 m fids for the 
 right - hand panel . ab initio calculations 
 confirm the experimental conclusion that the detected molecules are 
 indeed ag cl so determined are recorded in table 2 , together with the rotational 
 constants they imply for the various isotopologues investigated . for 
 comparison , the observed rotational constants are also included in 
 table 2 . cl isotopologues between 
 observed and calculated values no doubt arise because the observed 
 rotational constants are zero - point quantities while those calculated 
 refer to the hypothetical equilibrium geometry . calculated by fitting all available 
 moments of inertia with r(cc ) fixed at the 
 ab initio value . the quoted errors are those generated by assuming 
 a range of 0.02  about the assumed r(cc ) 
 value . although correlation 
 problems prevent fitting of zero - point principal 
 moments of inertia to determine independently all three distances r(m  c ) , r(cc ) , and r(c  cl ) necessary to define the r0 geometry of the linear molecules m  cc  cl 
 ( m = ag or cu ) , some progress is possible as follows . cl , r(cc ) 
 lies within 0.02  of the equilibrium value 1.2219  calculated 
 at the ccsd(t)/aug - cc - pv5z level of theory ( a reasonable assumption 
 given the high level of theory and the fact that the difference between r0 and re for the short , stiff triple bond in acetylene is 0.003  ) . the values that result from fitting the other two distances to all 
 six observed zero - point principal moments of inertia ( using the program 
 strfit of kisiel ) are then r(ag  c ) = 2.015(14 )  and r(c  cl ) 
 = 1.635(6 )  , where the errors quoted reflect the range of values 
 implied by the assumed range 0.02  in r(cc ) . these results are included in table 2 together with the corresponding r0 quantities , similarly determined , for cu cc  cl . while the individual r0 bond distances r(m  c ) and r(c  cl ) are not 
 well determined by this approach , the overall length of the molecule 
 is . the distance r(mcl ) between 
 the two outer atoms turns out to be very insensitive to the assumed 
 value of r(cc ) . for example , if we assign 
 the large error of 0.02  to the assumed distance r(cc ) = 1.2219  in ag  cc similar conclusions follow for cu  cc  cl , as 
 recorded in table 2 . a better approach 
 to the determination of the full geometry of 
 each molecule m cc  cl ( m = ag or cu ) would be 
 to obtain the rs coordinate of every atom 
 of the parent isotopologue by making isotopic substitutions at each 
 atom in turn . kraitchman provided a simple 
 equation relating the change be in the equilibrium rotational constant accompanying 
 isotopic substitution at an atom i in a linear molecule 
 and the equilibrium coordinate zie of the substituted atom i , namely1where be = be be 
 is the change in the rotational constant accompanying 
 the isotopic substitution ,  = mm/ ( m + m ) is the 
 reduced mass of the substitution , and m and m are the mass of the parent molecule and the mass change , 
 respectively . except for the simplest of linear molecules , much used alternatives 
 to equilibrium coordinates of atoms are the so - called rs quantities . the rs coordinate 
 of an atom i is defined by using zero - point rotational 
 constants in kraitchman s equations in place of equilibrium 
 values . the rs geometry has the advantage that it is closer to its re counterpart than the r0 variety ; 
 for example , it is readily shown that rs = ( r0 + re)/2 for a diatomic molecule . moreover , rs geometries show greater internal consistency when overdetermined 
 than do the r0 variety . cl were not observed for either m = ag or cu in the present 
 investigation , so a full rs geometry is 
 unfortunately not available for either molecule . nevertheless , 
 some measure of the agreement between the partial rs geometry and the re ( ab 
 initio calculated ) version is available for each m  cc  cl 
 from a comparison of the principal axis coordinates am and acl and , thereby , the 
 distances between the outer atoms m and cl obtained by the two methods . 
 the results from ag and cl single substitutions 
 made in the parent molecule ag  cc  cl are |aag| = 1.4064(11 )  , |acl| = 3.46645(43 ) 
  , respectively , and thence r(agcl ) 
 = 4.8729(12 )  , all of which are in good agreement with their 
 equilibrium counterparts obtained ab initio at the ccsd(t)/aug - cc - pv5z 
 level of theory and with the r0 versions 
 ( see table 2 ) . the corresponding rs quantities for the cu analogue are |acu| = 1.8172(8 )  , |acl| = 2.8571(5 )  , and r(cucl ) 
 = 4.6753(10) based on the cu cl isotopologue as parent , 
 and these too are in good agreement with the ab initio and r0 values ( see table 2 ) . cc  cl have been synthesized by a laser - ablation 
 method and were unambiguously characterized by means of their rotational 
 spectra . cl 
 are formed , but it is well - known that ccl , cccl , and ccl2 are among the products when a thermal plasma is sustained in ccl4/ar mixtures . the microwave rotational 
 spectra of cccl and ccl2 are known , and thereby we were able to establish the presence of both species 
 in the laser - ablated , supersonically expanded gas pulses in the experiments 
 described here . the approximate ratio of the amounts of agcccl , ccl2 , and cccl determined from their detected signals was 1.0:1.0:0.1 , 
 with a similar ratio appropriate for cucccl . the geometry of the  ground state of the cccl radical has r(c  c ) = 1.267  , r(c  cl ) = 1.634 
  , and cccl = 156.9. the first and third of these 
 lie midway between the corresponding values in ethyne and ethene ( see 
 ref ( 18 ) for the r0 geometries of hcch and ch2=ch2 ) , while r(c  cl ) = 
 1.634  is very similar to the corresponding distance determined 
 here for both ag c bonds . given 
 that the cccl radical is present together with ag / cu atoms ( and/or 
 ions ) , the formation of ag cc  cl and cu  cc  cl 
 from cccl by picking up a ag or the method described here promises a route to the synthesis of other 
 new , simple molecules .
the 
 new linear molecule ag  cc  cl has been 
 detected and fully characterized by means of rotational spectroscopy . 
 it was synthesized by laser ablation of a silver rod in the presence 
 of a gaseous sample containing a low concentration of ccl4 in argon , cooled to a rotational temperature approaching 13 
 k through supersonic expansion , and analyzed by chirped - pulse , fourier 
 transform microwave spectroscopy . 
 six isotopologues were investigated , 
 and for each the spectroscopic constants b0 , dj and aa(cl ) were determined . 
 the b0 values were interpreted to give the following bond lengths : r(ag  c ) = 2.015(14 )  and r(c  cl ) = 1.635(6 )  , with r(cc ) 
 = 1.2219  assumed from an ab initio calculation at the ccsd(t)/aug - cc - pv5z 
 level of theory . 
 the cu analogue cu 
 cc 
 cl was 
 similarly identified and characterized .