document: molecular hydrogen is the most abundant molecule in the universe and the main constituent of regions where stars are forming . 
   h@xmath2  plays an important role in the chemistry of the interstellar medium , and its formation governs the transformation of atomic diffuse clouds into molecular clouds . because of the inefficient gas phase routes to form h@xmath2 , dust grains have been recognized to be the favored habitat to form h@xmath2  molecules ( @xcite , @xcite ) . 
   the sticking of h atoms onto surfaces has received considerable attention because this mechanism governs the formation of h@xmath2 , but also other molecules that contain h atoms . 
   the sticking of h atoms onto dust grains can also be an important mechanism to cool interstellar gas ( @xcite ) . in the past decades 
   , a plethora of laboratory experiments and theoretical models have been developed to understand how h@xmath2  forms . 
   as h atoms arrive on dust surfaces , they can be weakly ( physisorbed ) or strongly ( chemisorbed ) bound to the surface . 
   the sticking of h in the physisorbed state ( @xcite , @xcite , @xcite ; @xcite ) and in the chemisorbed state ( @xcite ; @xcite ; @xcite ) has been highlighted by several experiments on different types of surfaces ( amorphous carbon , silicates , graphite ) .    in the ism , 
   dust grains are mainly carbonaceous or silicate particles with various sizes and represent an important surface for the formation of h@xmath2 
   . however , a large part ( @xmath3 50@xmath4 ) of the available surface area for chemistry is in the form of very small grains or pahs ( @xcite ) . 
   these pahs are predicted to have characteristics similar to graphite surfaces : however , once the first h atom is chemisorbed on the basal plane , subsequent adsorptions of h atoms in pairs appear to be barrierless for the para dimer and with a reduced barrier for the ortho dimer ( @xcite ) . 
   h@xmath2  can then form by involving a pre - adsorbed h atom in monomer ( @xcite ; @xcite ; @xcite ; @xcite ) or in a para - dimer configuration ( @xcite ) . however , while these routes represent efficient paths to form h@xmath2 , the inefficient sticking of h atoms in monomers constitutes an important obstacle to enter the catalytic regime for h@xmath2  formation . 
   this results in a very low h@xmath2  formation efficiency on graphitic / pah surfaces ( @xcite ) . 
   the hydrogenation on the pah edges has been identified as an important route to form h@xmath2  in the ism ( @xcite ; @xcite ; @xcite ; @xcite ; @xcite ) . 
   density functional theory calculations have shown that the first hydrogenation of neutral coronene is associated with a barrier ( @xmath360 mev ) but that subsequent hydrogenation barriers vanish ( @xcite ) . recently 
   , coronene films exposed to h / d atoms at high temperature , were studied by means of ir spectroscopy ( @xcite ) and mass spectrometry ( @xcite ) . 
   these measurements showed that neutral pahs , when highly hydrogenated , are efficient catalysts for the formation of h@xmath2 , and confirmed the high h@xmath2  formation rate attributed to pahs in pdrs ( @xcite ) . 
   pah cations , which are usually present at lower extinction a@xmath5 , and therefore reside at the surfaces of pdrs , also represent an important route to form h@xmath2  ( @xcite ; @xcite ) . 
   the addition of the first h atom is predicted to be barrierless . 
   this reaction is exothermic but the product should be stabilized by ir emission . 
   a second h atom can react with the already adsorbed h to form h@xmath2  without a barrier ( @xcite ; @xcite ) .    in this letter 
   , we study experimentally the hydrogenation of coronene cations in the gas phase through exposure to hydrogen atoms . by using mass spectrometry 
   , we show that odd hydrogenation states of coronene cations predominantly populate the mass spectrum . 
   our results highlight the fact that the further hydrogenation of pah cations is associated with a barrier if the number already attached h atoms is odd , and no barrier if this number is even . 
   this alternanting barrier - no barrier occurence seems to remain with increasing hydrogenation . 
   these results suggest that pah cations can also enjoy highly hydrogenated states in the interstellar medium , and acts as catalysts for h@xmath2  formation . 
   in this pilot experiment we show the feasibility of studying the hydrogenation of pahs in the gas phase . for this purpose , 
   we use a setup designed to study molecular ions in a radiofrequency ion trap . 
   time - of - flight mass spectrometry of the trap content is used to identify the changes in mass of the coronene cations and therefore deduce their respective degrees of hydrogenation . 
   the experiments have been performed using a home - built tandem - mass spectrometer shown schematically in figure [ fig : setup ] ( @xcite ) . 
   a beam of singly charged coronene radical cations ( [ c@xmath6h@xmath7@xmath8 , m / z 300 ) was extracted from an electrospray ion source . 
   the ions were phase - space compressed in an radiofrequency ( rf ) ion funnel and subsequently in an rf quadrupole ion guide . 
   mass selection was accomplished by using an rf quadrupole mass filter . 
   accumulation of the ions took place in a three dimensional rf ion trap ( paul trap ) . 
   a he buffer gas at room temperature was used to collisionally cool the trapped cations . 
   exposure to gas - phase atomic hydrogen for variable periods of time led to multiple hydrogen adsorption on the coronene cations . 
   an electric extraction field was then applied between the trap end - caps to extract the trapped hydrogenated cations into a time - of - flight ( tof ) mass spectrometer with resolution m/@xmath9 m @xmath3 200 . to obtain mass spectra of sufficient statistics , typically a couple of hundred tof traces were accumulated . 
   electrospray ionization allows to gently transfer ions from the liquid phase into the gas phase . 
   inspired by the method of @xcite we have run the ion source with a solution consisting of 600 @xmath10l of saturated solution of coronene in methanol , 350 @xmath10l of hplc grade methanol and 50 @xmath10l of 10 mm solution of @xmath11 solution in methanol . in the liquid phase , electron transfer from a coronene molecule to a silver ion 
   leads to formation of the required radical cation . 
   the trapped ions are exposed to hydrogen atoms produced from h@xmath2  by a slevin type source which has been extensively used in crossed beam experiments ( @xcite,@xcite ) . 
   while in the earlier work the dissociation fractions were determined by means of electron impact excitation or heii line emission , we now use charge removal ( captured ionization ) and dissociation induced by 40 kev he@xmath12 . for these processes 
   the cross sections are well - known ( @xcite ) . in this way 
   we determine a hydrogen dissociation fraction of @xmath13 . 
   the temperature of the h beam is around room temperature ( @xmath325  mev ) . 
   coronene ions are exposed to a constant flux of h atoms for different periods of time before their degree of hydrogenation is determined by means of mass spectrometry . the irradiation time is varied from 1.0 up to 30 s to study the time - dependence of coronene hydrogenation . 
   the data obtained from our experiment are a series of mass spectra of hydrogenated coronene cations as a function of h exposure time . 
   some of the spectra are shown in fig.[fig : rawdata ] . 
   fig.[fig : rawdata](a ) shows the mass spectrum of the native m / z=300 coronene cations . 
   a similar , thus unchanged , mass spectrum is obtained ( not shown in this article ) if we irradiate coronene cations with molecular hydrogen . 
   this means that molecular hydrogen does not stick to coronene cations at room temperature .    after turning on the hydrogen source and exposing the coronene cations to the atomic hydrogen beam for 1.0 s ( fig.[fig : rawdata ] , ( b ) ) , the peak at @xmath14 shifts to 301 , which means that the trap content main constituent is ( c@xmath6h@xmath15+h)@xmath8 . for increasing irradiation time ( fig.[fig : 
   rawdata](c ) t= 2 s , ( d ) 3 s , ( e ) 4 s and ( f ) 4.75 s ) , the peak at @xmath16=301 disappears progressively while a peak at @xmath17 and then at @xmath18 ( for t = 4.75 s see fig.[fig : rawdata](f ) ) appears , which indicates the addition of 3 and 5 hydrogen atoms , respectively . at longer exposure time ( fig.[fig : longdata](a ) 
   t @xmath315 s ) , the @xmath16=303 peak dominates the signal , and a peak at @xmath16=305 appears . at even longer irradiation times ( fig.[fig : longdata](b ) 
   t @xmath330  s ) , the peak @xmath16=305 dominates and peaks at @xmath16=307 and 309 appear . 
   these peaks clearly show the evolution of the hydrogenation states of coronene cations with h irradiation time . 
   our results show that the most important peaks measured in the mass spectrum shift from lower masses to higher masses with increasing h exposure time . in order to follow the evolution of the first hydrogenated state of coronene cation ( c@xmath6h@xmath15+h)@xmath8 ( corh@xmath8 ) to the second ( c@xmath6h@xmath15 + 2h)@xmath8 ( corh@xmath19 ) , third ( corh@xmath20 ) and fourth ( corh@xmath21 ) hydrogenated states 
   , we use a simple model that describes this evolution :    @xmath22    @xmath23    @xmath24    @xmath25    hydrogenation of corh@xmath26@xmath8 follows an arrhenius expression where a@xmath27 is the prefactor and e@xmath27 is the barrier , while hydrogenation of corh@xmath28 follows the same expression with a prefactor a@xmath26 and no barrier . 
   k@xmath29 is the boltzmann constant and t the temperature of the h beam ( t@xmath325  mev ) .    in these equations 
   we do not include abstraction , meaning that the time evolution of the contribution of each state is governed entirely by hydrogenation . 
   this assumption is made in order to derive the first barriers of hydrogenation . 
   abstraction can be neglected in the conditions of our experiments for low exposure times . 
   this is supported by previous experiments where the cross section for addition of hydrogen to neutral coronene is predicted to be 20 times that for abstraction ( @xcite ) . 
   further support is drawn from a kinetic chemical model we developed , which shows that abstraction has to be very low compared to hydrogenation to be able to mimic the experimental results ( boschman et al . in prep ) . 
   however , for long h exposure time we expect the hydrogenation degree of the coronene cations to reach a steady state which will allow us to derive the contribution of abstraction relative to addition , and therefore derive the h@xmath0 formation rate due to pah cations . 
   it should also be kept in mind that in the conditions of our experiments , the h atoms are at room temperature meaning that they cross the barriers for abstraction ( 10 mev , @xcite ) and addition ( 40 - 60 mev , @xcite ) with similar ease . under interstellar conditions , however , the abstraction will dominate by 8 orders of magnitude ( at 20 k ) because of the barrier differences .    the first hydrogenation is expected to take place at the outer edge carbon atom ( @xcite ) . 
   this state provides more conformational freedom to the four neighbouring outer edge carbon atoms , ensuring a preference for the second hydrogenation to take place at one of those four carbon atoms . 
   the third hydrogenation will preferentially take place at the outer edge carbon next to the second h atom . 
   again , the forth h atom can be bound to one of the four neighbouring outer edge carbon atoms , and the fifth sticks on the neighboring outer edge carbon . 
   this scenario of h atoms sticking preferentially on outer edge carbons next to already adsorbed atoms is described in @xcite . 
   the contribution of every peak is determined by fitting our data with gaussians with identical widths ( see fig.[fig : fit](a ) ) . 
   the ratios between different hydrogenation states as function of time are reported in fig.[fig : fit](b ) . 
   it appears that the ratio between the contribution of the first ( corh@xmath8 ) and the second ( corh@xmath19 ) hydrogenation state does not evolve with time for short time scales @xmath30 . 
   also , the ratio between the third ( corh@xmath20 ) and the forth ( corh@xmath21 ) hydrogenation state shows identical behaviour after t@xmath31 2s @xmath32 . before this exposure time 
   the n@xmath33 and n@xmath34 signals are very weak , and the ratio is uncertain . 
   we can therefore assume that for these measurements @xmath35 and @xmath36 . 
   the expression for the corh@xmath37 to corh@xmath38 as well as for the corh@xmath39 to corh@xmath40 energy barriers can then be written as :    @xmath41    @xmath42    from these expressions we derive the energy barrier e@xmath2 as 72@xmath16 mev and e@xmath43 as 43@xmath18 mev , as shown in fig.[fig : fit](c ) . 
   this shows that hydrogenation barriers are decreasing with increasing hydrogenation . 
   however , our results also show that odd hydrogenated states dominate the mass spectrum even for high degrees of hydrogenation ( fig.[fig : longdata ] ) . 
   this highlights the presence of a barrier - no barrier alternation from one hydrogenated state to another , up to high hydrogenation states . 
   so our results indicate that even if the hydrogenation barriers decrease for the first hydrogenations , they do not vanish completely and remain at higher hydrogenation states . 
   the barriers derived in our study are similar to the one calculated by @xcite for neutral coronene . 
   this means that the first hydrogenations of coronene cations should be comparable to the hydrogenation of neutral coronene . 
   however , for higher degree of hydrogenation we show that these barriers still exist , while the calculations from @xcite predict that these barriers vanish after a few hydrogenations . 
   recent mass spectrometric measurements of coronene films exposed to h / d atoms do not show preferences for even or odd hydrogenation states of neutral coronene ( @xcite ) . 
   however , these measurements are not very sensitive to barrier heights well bellow 100 mev , since the experiments were performed with atoms at beam temperature of 170 mev .    in pdrs exposed to uv fields less than few hundreds g@xmath44 , the spatial distribution of h@xmath2  and pahs does correlate ( @xcite , @xcite , @xcite ) contrary to what is seen in the presence of strong uv fields ( @xcite , @xcite ) . 
   the h@xmath2  formation rates have been derived for several pdrs exposed to various uv radiation fields . 
   these rates can be explained by the contribution of pahs to the formation of h@xmath2  ( @xcite ) . 
   depending on the uv intensity , the pahs observed can either be pah cations , that are present in regions at low visual exctinctions a@xmath5 , or neutral pahs , which are located at higher extinctions 
   . work by @xcite and @xcite has shown that high - uv and high density pdrs ( n@xmath45@xmath3110@xmath46 @xmath47 and g@xmath44@xmath31100 , g@xmath48 ) can maintain a @xmath3 30@xmath4 cationic fraction upto a few mag in a@xmath5 . 
   more relevant to this work , @xcite have studied low - uv pdrs ( g@xmath44@xmath49100 ) , and followed the pah charge balance for different densities , uv radiation fields and metallicities . 
   they found that pah cations dominate over neutrals and anions for a@xmath5@xmath492 mag . 
   the h@xmath2  formation rates observed in pdrs exposed to different uv fields can therefore be partly attributed to neutral and cationic pahs . 
   our results show that the hydrogenation processes of neutral and cationic pahs are similar and should contribute similarly to the formation of h@xmath2 . 
   further experimental investigations will allow us to derive the h@xmath2  formation rate for pah cations . 
   we have investigated the addition of hydrogen atoms to coronene cations in the gas phase and observed increasing hydrogenation with h exposure time . 
   our results show that odd hydrogenated states dominate the mass spectrum , which evidences the presence of a barrier for the further hydrogenation of odd hydrogenation states . 
   the first hydrogen sticks to the coronene cations without a barrier ( @xcite , @xcite ) . 
   the second and forth hydrogenations are associated with barriers of about 72 @xmath1 6 mev and 43 @xmath1 8 mev , while the third and fifth hydrogenation are barrierless . 
   these barriers are similar to the one calculated for neutral coronene ( @xcite ) . 
   our results indicate that superhydrogenated pah cations ( @xcite ) should also be found in the interstellar medium , and be important catalysts for the formation of h@xmath2 , as it is the case for their neutral counterparts . 
   l. b. and s. c. are supported by the netherlands organization for scientific research ( nwo ) . 
   g.r . recognizes the funding by the nwo dutch astrochemistry network . 
   we would like to thank the anonymous referee for the helpful comments .
summary: molecular hydrogen is the most abundant molecule in the universe . a large fraction of h@xmath0 forms by association of hydrogen atoms adsorbed on polycyclic aromatic hydrocarbons ( pahs ) , where formation rates depend crucially on the h sticking probability . 
   we have experimentally studied pah hydrogenation by exposing coronene cations , confined in a radiofrequency ion trap , to gas phase atomic hydrogen . 
   a systematic increase of the number of h atoms adsorbed on the coronene with the time of exposure is observed . 
   odd coronene hydrogenation states dominate the mass spectrum up to 11 h atoms attached . 
   this indicates the presence of a barrier preventing h attachment to these molecular systems . for the second and fourth hydrogenation , 
   barrier heights of 72 @xmath1 6 mev and 40 @xmath1 10 mev , respectively are found which is in good agreement with theoretical predictions for the hydrogenation of neutral pahs . 
   our experiments however prove that the barrier does not vanish for higher hydrogenation states . 
   these results imply that pah cations , as their neutral counterparts , exist in highly hydrogenated forms in the interstellar medium . due to this catalytic activity 
   , pah cations and neutrals seem to contribute similarly to the formation of h@xmath0 .