4-aminobenzonitrile 
 ( abn ) can be categorized as a para - substituted 
 benzonitrile ( figure 1 ) . this class of molecule 
 has received much attention because some display dual fluorescence , 
 i.e. , the solvent - dependent appearance of a second emission at a different 
 frequency from a charge - transfer state upon local excitation . this difference was explained by the additional electron - donating 
 character of the alkylated amino group in dmabn . thus , the electronic environment at the amino - group nitrogen 
 can be decisive for the existence of charge - transfer states and hence 
 dual fluorescence ( see , for example , refs ( 9 ) and ( 10 ) ) . the nitrogen atoms of 
 the nitrile 
 and the amino group lead to a complicated hyperfine structure of rotational 
 transitions . the principal inertial axes are labeled as a , b , and c. here , the rotational spectrum of abn in the frequency range 
 28.5 
 ghz measured by broadband microwave spectroscopy is reported . the 
 two chemically distinct nitrogen atoms of the respective amino and 
 nitrile groups of this molecule lead to a hyperfine structure of each 
 rotational transition due to nuclear quadrupole coupling . determining 
 their individual quadrupole interactions is challenging , due to the 
 complicated , combined splitting effect of the two nuclei . they allow 
 for the investigation of the electronic environment of the molecule 
 in the vicinity of the nitrogen atoms , which is of particular interest 
 when abn is compared to other para - substituted benzonitriles showing 
 dual fluorescence . previous microwave spectroscopy studies investigated 
 the differences between dimethylaniline ( dma ) and dmabn . here the rotational constants of the asymmetric rotor abn 
 were determined 
 earlier from rotationally resolved electronic spectra . due to the lower resolution and higher rotational transitions , and the frequency range of our spectrometer 
 ( 28.5 ghz ) together with the high resolution provided by microwave 
 spectroscopy facilitates their identification . the two nonvanishing 
 dipole moment components of abn are 5.46 d along the direction of 
 the weakest moment of inertia ( a ) and 1.45 d in the direction orthogonal to the benzene ring ( c ) ; hence the 
 spectrum is dominated by a - type asymmetric rotor transitions . abn is also a potential candidate for extending the ongoing molecule - deceleration 
 experiments in our group toward larger molecules . we recently demonstrated 
 that the interaction of polar molecules with strong external microwave 
 fields can be used to manipulate their motion , e.g. , to guide , focus , 
 and decelerate them . the force on the molecules results 
 from the ac stark effect of the particular rovibronic states of interest . as it is planned to use abn in our beam - slowing experiments , a sound 
 knowledge of its rotational spectrum is necessary . abn is especially 
 suited for this purpose because of its advantageous dipole moment - to - mass 
 ratio and its efficient detection after ionization with a mass spectrometer , 
 which is characterized in this work as well . first , we present our results 
 on the nitrogen hyperfine splitting of abn and thus extend recent 
 studies . second , we want to present our 
 new apparatus , a broadband microwave spectrometer equipped with a 
 laser ablation source and a time - of - flight ( tof ) mass spectrometer . the data presented in this work was recorded in our 
 new spectrometer , 
 which is described here for the first time . it was constructed in 
 a modular manner and consists of three vacuum cross - chambers : one 
 for the molecular source , one for the microwave excitation / detection , 
 and one housing a tof mass spectrometer for beam diagnostics . in the source chamber the molecules of interest are either coexpanded 
 with a carrier gas through a pulse nozzle or seeded into the expanding 
 carrier gas by ablating them with a laser pulse from a solid sample . 
 the latter method is especially suited for fragile molecules . to measure 
 rotational spectra , the fourier - transform microwave spectroscopy ( ftmw ) 
 technique with a broadband chirped - pulse our accessible frequency range of 28.5 
 ghz is particularly interesting for larger molecular systems like 
 biomolecules and complexes , which have large moments of inertia and 
 thus small rotational constants . consequently , the main part of the 
 rotational spectrum is located in this low - frequency range . furthermore , 
 mainly transitions between low - lying rotational states are observed 
 for molecules of the size of abn . this is advantageous for determining 
 the nuclear quadrupole coupling constants of weakly coupling nuclei , 
 such as nitrogen , because the arising splittings scale inversely with 
 the rotational quantum number j. in addition 
 to the microwave spectrometer , a commercial tof ion 
 mass spectrometer ( jordan tof products ) of wiley  mclaren type was implemented in the experiment . together 
 with an ionization laser , this is a highly sensitive tool for monitoring 
 and analyzing our molecular beam and particularly valuable for gaining 
 information about the formation of complexes . the combination with 
 the microwave capabilities of the apparatus will also allow for double - resonance 
 experiments . furthermore , by scanning the frequency of the ionizing 
 laser pulse the vibrational structure of electronic levels can be 
 investigated , as shown below for abn . for the experiments described 
 here , abn was purchased from sigma - aldrich 
 ( 98% purity ) and used without further purification . the sample was 
 heated to 113 c in a reservoir and the carrier gas neon , with 
 a backing pressure of 2.5 bar , was flowed over it to create a gas 
 mixture . subsequently , this mixture was expanded into vacuum through 
 a pulse nozzle with an opening time of approximately 400 s 
 and a repetition rate of 2 hz . after an expansion time of 775 s , 
 the abn molecules were excited by a 1 s long microwave pulse , 
 which was linearly frequency chirped from 2 to 8.5 ghz . the resulting 
 free induction decay ( fid ) was recorded for 50 s starting 5 
 s after the excitation pulse and then amplified and converted 
 into a digital signal . in total 337  000 fids were recorded 
 and digitally averaged . the rotational spectrum was obtained by fourier 
 transforming the experimental data using a fast fourier - transform 
 algorithm , without any filters or other treatments . the length of 
 the recorded fid set our frequency resolution to 20 khz , whereas the 
 full width at half - maximum of our transitions was on the order of 
 30 khz . the spectrum of abn is depicted in figure 2 , together with the results of a fit ( see later ) the upper trace shows 
 the experimental data , and the lower trace represents the result of 
 fitting an asymmetric rotor hamiltonian to it . note that a number of 
 unassigned lines remain that can be attributed 
 neither to the abn monomer nor to its isotopologues . as mentioned , the experimental setup allows 
 us to analyze the molecular beam with the tof mass spectrometer . the 
 recorded mass spectrum is depicted in figure 3 and confirms the existence of abn complexes . besides the dominant 
 monomer peak , at the mass of 118.4 g / mol , the dimer , trimer , and tetramer 
 of abn are clearly visible . also the complex of the abn dimer with 
 water could be identified . interestingly , no complex of the abn monomer 
 and water was observed . to obtain the mass spectrum of abn , 
 the molecules were first ionized 
 with a 8 ns long light pulse with a pulse energy of 6 mj from a tunable 
 nd : yag - pumped dye laser ( featuring a line width of 0.06 cm at a wavelength of 570 nm ) . the resulting ions were accelerated 
 in an electric field gradient , then they flew through a 1 m long drift 
 tube and finally they were detected by a multichannel plate upon their 
 arrival at the end of the tube . the tof between excitation and detection 
 depends on their mass - to - charge ratio and thus , assuming single ionization , 
 allows for the determination of their mass . during the experiment , the ionization laser frequency was kept resonant for the abn monomer 
 at a wavelength of 291.5 nm . even upon complex formation the resonances 
 do not shift much , so that ionization of the complexes was possible . 
 for the final spectrum , ion fragments from the molecular beam of abn analyzed 
 with the 
 tof mass spectrometer . for each point 36 000 acquisitions were averaged . with the same setup it is possible 
 to study the vibrational structures 
 of electronic levels by a combination of tof mass spectrometry and 
 ionization techniques , such as resonance enhanced multiphoton ionization 
 ( rempi ) . to demonstrate its functionality and to test the detection 
 efficiency of abn using rempi for our upcoming beam - slowing experiments , 
 the one - color two - photon ionization spectrum of abn in the range from 
 290 to 300 the different 
 lines represent resonances of different vibrational levels of the 
 same excited electronic level . the origin was set to the known value 
 of 33  493 cm , and the line assignment followed 
 previous work using varsnyi notation . the frequency was varied in steps of 0.01 nm and 
 for each point 
 180 acquisitions were averaged . the line positions were determined 
 by fitting a lorentzian line shape to each peak . their assignment 
 was carried out in agreement with refs ( 9 ) and ( 20 ) and also agrees well with more recent results ( table 1 ) . in the following , the analysis of the microwave 
 spectrum of abn 
 and especially the results on the nuclear quadrupole coupling in this 
 molecule are discussed in more detail and compared to data for related 
 systems . the differences to dmabn are highlighted and presented with 
 the help of analyzing the occupation numbers of the atomic orbitals 
 of the nitrogen in the amino group . the microwave spectrum was 
 fitted using an asymmetric rotor hamiltonian 
 ( watson s - reduction in i representation ) as implemented in the program pgopher . rotational quantum numbers involved 
 in the assigned transitions range from j = 1 to j = 4 . for the corresponding 
 energy levels the influence of distortion constants was found to be 
 small and thus have been neglected in the fitting procedure . as expected 
 from the dipole moment components , the spectrum of abn is dominated 
 by a - type transitions . despite the still significant 
 dipole moment along the c direction , no corresponding 
 transitions were observed . also no signatures from c 
 or other isotopologues of the molecule they show good agreement with the 
 previously reported rotational constants and predictions of ab initio 
 calculations . concerning the quantum chemical methods , the overall 
 best agreement is obtained by density functional theory using the 
 m06 - 2x functional and a 6 - 31+g(d , the differences in 
 the results obtained at the mp2 level of theory mainly arise from 
 different equilibrium positions of the hydrogen atoms in the amino 
 group . note that no line splitting arising 
 from the inversion motion of 
 the amino group is observed under the cold conditions of a molecular 
 jet . similar to the case for the related molecule aniline , the splittings 
 of the respective energy levels are so large that the thermal population 
 of the excited modes becomes negligible at low temperatures ( compare , 
 e.g. , refs ( 26 ) and ( 27 ) ) . the nuclear quadrupole 
 splitting of the rotational transitions 
 arising from the different nitrogen nuclei is nicely resolved for 
 the two lower rotational transitions ( j + 1  j = 3  2 and 2  1 ) , whereas for the transitions 
 with j + 1  j = 4  
 3 the groups of lines become quite congested . nuclear quadrupole coupling splitting due to the two nitrogen atoms 
 in abn for three different rotational transitions . for higher rotational 
 quantum numbers the splitting is smaller and the transition groups 
 are more congested and more difficult to resolve . the nuclear quadrupole coupling in molecules is very sensitive 
 to the local electric field gradient , and a comparison of the results 
 with a series of related molecules highlights their differences . to 
 relate the nuclear quadrupole coupling constants measured in the inertial 
 principal axis system of abn ( a , b , c ) to other molecules , the nuclear quadrupole 
 coupling tensors have to be transferred to their principal axis system 
 ( x , y , z ) . in the 
 case of the molecules described herein , the transformation between 
 the two coordinate systems is given by a rotation around the b axes by an angle .(11 ) because the off - diagonal elements of the nuclear 
 quadrupole coupling tensors are not determined in the experiment , 
 this angle was estimated from quantum chemical calculations at the 
 m06 - 2x/6 - 31+g(d , p ) level of theory . for this purpose the eigenvectors 
 of the calculated nuclear quadrupole tensor were expressed in the 
 coordinates of the inertial principal axis system . is then 
 given by the angle between the axes x and a or equivalently between the axes z and c. the axes y and b coincide . the transformed values of the nuclear quadrupole coupling components 
 are then derived from the experimental results by a coordinate rotation , 
 as discussed in ref ( 11 ) . for the nuclear quadrupole coupling constants of the nitrile - nitrogen 
 atom , the corresponding corrections are very small and thus have been 
 neglected . in table 3 , the obtained nuclear 
 quadrupole coupling constants for abn , dmabn , and benzonitrile are 
 compared . both the amino and the dimethylamino group are known to 
 have a strong electron - donating effect on the aromatic ring . the very 
 similar nuclear quadrupole coupling constants for abn and dmabn compared 
 to that of the reference molecule benzonitrile confirm this result . the comparable 
 nuclear quadrupole coupling constants of the amino 
 nitrogen resulting from these rotations are presented in table 4 . in molecules with methylated amino groups ( dmabn , 
 dma ) , the nuclear quadrupole coupling of the amino - nitrogen atom is 
 enhanced because of the electron - donating character of the methyl 
 groups . this property leads to a reduction of the amine nuclear quadrupole 
 coupling , as can be seen for dmabn when compared to dma . together these results indicate a lower electron density 
 close to the nitrogen atom in the amino group and thus a lower tendency 
 for the existence of charge - transfer states . in addition , the similarity 
 between abn and aniline is strengthened by the comparison in table 4 . values are given in the principle 
 axis system of . to further 
 support this argument , a population analysis of the 
 amino - nitrogen p - orbitals was carried out according to the townes dailey 
 model and in close analogy to refs ( 11 ) and ( 32 ) . it allows for the determination 
 of the following indicators from the nuclear quadrupole coupling constants : i(nx ) , the ionic character of the amino - group 
 n h bonds , i(nc ) , the ionic 
 character of the n  c bond between the amino group and the aromatic 
 ring , and c(nc ) , the covalent character of the latter . 
 from these quantities the orbital occupancies of the nitrogen p - orbitals 
 ( nx , ny , nz ) were calculated . note that nz corresponds to the lone - pair 
 occupation of the nitrogen atom . h bonds  is linked to the amount of s - type hybridization 
 and thus to i(nx ) ( see refs ( 30 ) and ( 33 ) , page 234 ff ) . its value 
 is needed for the computations above and was taken from quantum chemical 
 calculations for abn and from the corresponding references for the 
 compared molecules . note that in ref ( 11 ) , discussing dma and dmabn , there is some confusion 
 with the coordinate system labels , so that our values deviate from 
 the published ones . the interpretation of the resulting numbers , 
 listed in table 5 , follows closely the arguments 
 given in ref ( 11 ) . it stands out that abn 
 has the lowest occupation of the lone - pair orbital nz of the amino - nitrogen and a high covalent 
 character c of the amino - group bonds . the combination 
 of the high ionic character of the nitrogen aromatic - carbon bond i(nc ) in abn and the low ionic character 
 of this bond in dmabn suggests that less charge is concentrated at 
 this location . this could represent a barrier for charge - transfer 
 states and thus explain the absence of dual fluorescence in abn . in 
 general , this finding adds microscopic details of the electron distribution 
 to the discussion of dual emission in aminobenzonitrile - like molecules , 
 which can be related to the dynamics of the molecule upon electronic 
 excitation . we investigated the broadband rotational 
 and the rempi spectrum 
 of abn with our new experimental apparatus , which combines the power 
 of a broadband microwave spectrometer with the analytic advantages 
 of a mass spectrometer . the obtained information on vibronic transitions 
 is particularly interesting with respect to future beam - slowing experiments . the assigned rotational constants were determined to higher precision 
 and agree very well with previous findings from rotationally resolved 
 electronic spectra . we find that density functional theory 
 using the m06 - 2x functional showed the best agreement for abn . the hyperfine structure due to the nuclear quadrupole coupling 
 of the two chemically distinct nitrogen atoms in abn was resolved . the experimentally determined nuclear quadrupole coupling constants 
 allowed for the analysis of the electronic environment of the nitrogen 
 locations . it was found that dmabn and abn have a similar electronic 
 environment at the nitrile - nitrogen atom and hence that the influence 
 of the methyl groups through the aromatic ring is negligible compared 
 to the effect of the amino group itself . by calculating the 
 occupancies of the hybrid orbitals of the amino - group 
 nitrogen , we showed that there is a significantly lower charge density 
 in abn compared to other para - substituted benzonitriles like dmabn . this result is particularly interesting with respect to the occurrence 
 of dual fluorescence because it can be interpreted as a possible barrier 
 to charge - transfer states .
the 
 rotational spectrum of 4-aminobenzonitrile in the gas phase 
 between 2 and 8.5 ghz is reported . due to the two chemically distinct 
 nitrogen atoms 
 , the observed transitions showed a rich hyperfine structure . 
 from the determination of the nuclear quadrupole coupling constants , 
 information about the electronic environment of these atoms could 
 be inferred . 
 the results are compared to data for related molecules , 
 especially with respect to the absence of dual fluorescence in 4-aminobenzonitrile . 
 in addition , the two - photon ionization spectrum of this molecule was 
 recorded using a time - of - flight mass spectrometer integrated into 
 the setup . 
 this new experimental apparatus is presented here for the 
 first time .