Multiphoton ionization of aniline, aniline-15N and aniline-2,3,4,5,6-d5: ionization and fragmentation mechanisms

International Journal of Mass Spectrometry and Ion Processes, 99 (1990) 169-190

169

Elsevier Science Publishers B.V., Amsterdam

MULTIPHOTON IONIZATION OF ANILINE, ANILINE-“N AND ANILINE-2,3,4,5,6-d,: IONIZATION AND FRAGMENTATION MECHANISMS

JEFFREY

A. ZIMMERMAN*

and REBECCA M. O’MALLEY**

Chemistry Department, University of South Florida, Tampa, FL 33620 (U.S.A.)

(First received 29 September 1989; in final form 11 May 1990)

ABSTRACT We report the resonant and non-resonant multiphoton ionization (MPI) spectra of aniline, aniline-15N and aniline-2 ,3I4 75,6-d, obtained at wavelengths of 266,355 and 532 nm. Consideration of plots of fragment ion currents vs. laser pulse energy and analysis of the contributions from deuterated ions to total fragment ion intensities (obtained using aniline-2,3,4,5,6-d,) have allowed characterization of the mechanisms involved in the overall ionization/fragmentation process at the three wavelengths. The resonant MPI at 266 nm is found to take place via initial molecular ion formation, followed by photofragmentation of this ion. In both the non-resonant cases the first step appears to be a rapid dissociation of the neutral molecule into smaller neutral entities. These are subsequently ionized, and photofragmentation of the ions produced then takes place. At 355 nm a minor contribution to the ionization/dissociation mechanism appears to involve molecular ion formation followed by photofragmentation. INTRODUCTION

Multiphoton ionization (MPI) fragmentation patterns have been found to vary tremendously, depending on laser wavelength, pulse energy, pulse length and on the laser focusing conditions used in a particular experiment [l-4]. These observations have led to the conclusion that the ionization/fragmentation process takes place via different mechanisms which depend on experimental conditions. In this study we have recorded resonant and nonresonant MPI fragmentation patterns of aniline and two labelled anilines (aniline-“N and aniline-2,3,4,5,6-d,) over a range of laser pulse energies so we could use the labelled “N and D atoms to follow what happens to certain atoms in the molecule during the ionization/fragmentation process. Since the first observations that the ion current produced in an MPI experiment could consist not only of molecular ions but also of a collection * Present address: Chemistry Department, University of Florida, U.S.A. **Author to whom correspondence should be addressed. 0168-l 176/90/$03.50

0 1990 Elsevier Science Publishers B.V.

Gainesville,

FL 32611,

170

of fragment ions, there has been considerable speculation about the mechanism(s) by which the ions are produced. Several early reports in the literature favored the formation of the ions via an ionic fragmentation mechanism [2,5,6]. In 1982, Dietz et al. [7] proposed an ionic ladder-switching model in which the molecular ion is formed as soon as enough photons are absorbed to exceed the ionization energy of the neutral molecule. Then photon absorption switches to the molecular ion which produces fragment ions as the fragmentation reactions become energetically feasible. Once formed, fragment ions absorb photons and undergo photodissociation. MPI spectra for benzene, computed using this model, agreed well with those obtained experimentally. However, experimental evidence does exist to indicate that the ionic ladder-switching model may not be strictly followed under all experimental conditions. For example, Stiller and Johnston [8] demonstrated that, in at least one system, further photon absorption by the molecular ion could compete effectively with a low energy ionic fragmentation route. At about the same time as the proposal of the ionic ladder-switching model it became clear that an ionic fragmentation model of MPI was not sufficient by itself because in some systems (mainly organometallics) fragmentation of the neutral molecule was found to be the first step in the overall process. In a review article Gedanken et al. [9] surveyed the then available experimental data and concluded that there are three distinct types of mechanism by which the MPI process takes place. They categorized these as class A, class B and class C. All resonant multiphoton absorption is proposed to be class A or class B, depending on whether the molecule, once excited to a neutral resonant state, either absorbs more photons to form a superexcited state of the neutral molecule (lying just above the energy corresponding to the ionization energy) which autoionizes to form the parent ion, (class A) or it decomposes into neutral fragments (class B). The parent ion or neutral fragments can then absorb more photons to fragment further or ionize. Simply, the difference can be described as follows: class A behavior is ionization followed by fragmentation; class B behavior is fragmentation followed by ionization. (It should be noted that the ionic ladder-switching model proposed by Dietz et al. closely parallels the defined class A behavior and therefore can be considered to be included in this overall scheme.) Non-resonant MPI was also covered by Gedanken et al. who characterized it as class C behavior and suggested that it involves the coherent absorption of a large number of photons to populate a very high lying superexcited state of the neutral molecule, where autoionization to various excited ionic states (which may or may not fragment further) takes place in competition with fragmentation to neutral species. Gedanken’s classification scheme and model does appear to accommodate much of the experimental data available at present. However, there have been few, if any, studies in which direct comparisons of resonant and non-resonant

171

MPI have been made. In terms of his model this leads to some interesting considerations. It is well known that as the internal energy of a reacting ionic species is increased, the probability of straight cleavage reactions increases in comparison with the probability of rearrangement reactions [lo]. Therefore, at high laser pulse energies or under non-resonant MPI conditions, we would expect rapid up-pumping of the molecular ion or molecule to produce highly energetic species which dissociate after only a few vibrations, yielding little or no molecular rearrangement. However, at lower laser pulse energies or under resonant MPI conditions, where the precursor ion or molecule contains less internal energy, we would expect rearrangement reactions to be more favored. In this study we report the MPI fragmentation patterns of aniline, aniline15Nand aniline-2 93P4 9596-d, under both resonant and non-resonant conditions using wavelengths of 266, 355 and 532 nm. The 266 nm photon (4.67 eV) is close to resonance with the first excited singlet of the aniline molecule [l I] and since the ionization energy is 7.7eV [12], ionization at this wavelength takes place via a 1 + 1 resonant path. In contrast, the first 355 nm photon (3.49 eV) is not resonant with any known transition, but the second photon populates a broad absorption band associated with Rydberg transitions [l 11.It takes a total of three 355nm photons to ionize the molecule. The absorption of the first 532 nm photon (2.34 eV) is also a non-resonant process, but absorption of the second photon populates the first excited singlet (the same as that populated by one 266nm photon). The third 532nm photon takes the molecule to the same energy reached by two 355 nm photons and the fourth photon takes the molecule above its ionization energy. The MPI fragmentation patterns of the labelled anilines at the three wavelengths are examined to determine the extent of intramolecular atom movement (rearrangement) and the possible role of ionic ladder-switching. Using these data in conjunction with information on the laser pulse energy dependence of the fragment ion intensities, we are able to draw conclusions about the mechanism for the ionization/fragmentation process at each wavelength. EXPERIMENTAL

Apparatus

We obtained all the experimental data presented here using a time-of-flight mass spectrometer (TOFMS) which has been modified so that ions formed in the source of the instrument by electron impact or by laser-initiated multiphoton processes can be mass analyzed. Laser light at 532, 355 and 266 nm was produced from an Nd:YAG laser using harmonic generating crystals. Full operational details of the equipment have been published previously [3]. Data from between 200 and 500 laser pulses were averaged to

172

obtain the spectra reported here, which were digitized using a 1Ons time interval. All laser spectra were recorded using a time-lag [ 131value of 0.7 ps, this value giving optimal resolution over the recorded mass range. Chemicals

The aniline used was research grade. Aniline-d, (98%) and aniline-15N (99’/,) were obtained from M.S.D. Isotopes, St. Louis, MO [14]. All were used without further purification and were subjected to several freeze-pump-thaw cycles before introduction into the mass spectrometer via a needle valve. The integrity of the labelling in the aniline-d, was checked by NMR and it was found to contain no significant ring-attached hydrogen or nitrogen-attached deuterium. Procedure

To identify the nitrogen- and deuterium-containing fragment ions we needed to compare the fragmentation patterns of all three anilines obtained under identical experimental conditions. Spectra of aniline-d,, aniline-15N and aniline were therefore recorded over a range of laser pulse energies in consecutive runs on the same day under the same experimental conditions. The pressure and other experimental parameters were kept as constant as possible throughout all three sample runs, Peak heights were measured and entered in a computer program which calculated the percentage total ion current (% TIC) for all m/z values. The contributions from nitrogen-containing ions in aniline were calculated by comparison of the aniline and aniline-15N spectra. For example m/z 26 in aniline could be C,H:. or CN+ . In the aniline-“N spectrum the peak due to CN+ shifts one mass unit to m/z 27 (C15N+) leaving m/z 26 to contain only C,Hl’ . Once the % TIC is known for C,Hl ’ , the % TIC for CN+ in the unlabelled aniline can be calculated by subtraction. Calculating the contributions from deuterated ions was more complicated because the number of possible ions is greatly increased when using aniline-d, and each individual m/z value must be examined for possible deuterated ions, For example, m/z 40, which in aniline could be C,H: ’ or C,NH: , is shown by the “N labelling to be approximately 100% CZNHl, but in aniline-d,, m/z 40 can also be C, DH: (a C,H: -type ion) or C, Dl’ (a C3H: ’ -type ion). Because of these difficulties, some of the % TICS calculated for deuterated ions are given as a range of values. All other quoted values for deuterated ions have uncertainties estimated at about f 10%. For these calculations, contributions due to 13Cwere neglected.

173 RESULTS AND DISCUSSION

General comparison of electron beam, 266nm, 355nm and 532nm MPI

Figure 1 shows for comparison purposes a 70eV electron beam spectrum and MPI spectra obtained at the three wavelengths. The fragmentation patterns in the MPI spectra alter considerably as the laser pulse energy is increased (details will be presented and discussed later). However, the three MPI spectra shown here were obtained at the limits of detection of the mass spectrometer using the lowest laser pulse energies possible. The 70eV spectrum and the 266 nm MPI spectrum look very similar; in both spectra, the base peak is the molecular ion, C,H,NH:’ (m/z 93) and the most intense fragments are detected at m/z 66 (C,H,+‘) and m/z 39 (C,H:). Two differences are apparent. The peak at m/z 46.5 in the electron beam spectrum corresponds to the doubly charged molecular ion. No doubly charged ions are detected in the 266 nm spectrum or in any of the MPI spectra. (This is true for all the systems we have studied.) Also the low mass fragment ion intensities are greater in the 266 MPI spectrum than in the 70eV electron beam. In contrast, the 532 nm and 355 nm spectra are quite different from the other two. The molecular ion is either small or non-existent and the majority of the ion intensity is observed as low mass ions: C+‘, C:. , CNH: , C,H: and C, NH; . Electron beam Nitrogen-containing ions

A comparison with the corresponding 70eV aniline-“N spectrum (not shown) indicates that in addition to the molecular ion the nitrogen-containing ions are m/z 28 (65% CNHZ), m/z 40 (100% C,NHl), m/z 41 (100% C,NHc’), m/z 52 (100% C3NHl), m/z 54 (100% C,NHi) and m/z 67 100% (C,NH:‘). However, the majority of the fragment ions (80%) contain only hydrogen and carbon atoms, indicating that under these ionizing conditions most of the ionic fragments contain atoms originating from the ring. Deuterium-containing ions

Table 1 summarizes the % TIC for the deuterated ions which correspond to the four most intense fragment ions in the aniline spectrum. We make two significant observations. Firstly, it is interesting that in aniline-d,, 90-100% of the C,Hz ’ is formed as C,DgH+’ and only a very small amount (O-10%) is formed as C,D,H,f ’ . This is in contrast with our previously reported 20 eV data [15] which showed approximately 25% of C,H,+ ’ being formed as that is, containing both the original amine hydrogen atoms. An C,D,HL explanation for this may be found in some work reported by Flammang et al.

174 C6H5NHzt

a)

m/z

10

20

30

40

50

60

70

60

90

C6H5NH2t

b)

m/z

lb

c)

10

30

40

50

60

70

60

90

CNH2+

C3NH;

ml2

10

20

30

40

50

60

70

60

90

20

30

40

50

60

70

60

90

C?

d)

ml2

10

Fig. 1. Aniline: (a) 70 eV electron 355nm MPI, l.lmJ.

beam; (b) 266 nm MPI, 0.1 mJ; (c) 532 nm MPI, 5 mJ; (d)

175 TABLE 1 The % TIC of deuterated ions formed using 70eV electron impact Ion in aniline

mlz

% TIC

Ion(s) in aniline-d,

mlz

Contribution

CSH,+’

66

12.3

GH:

65

6.1

GH:

39

6.7

CNH:

28

2.2

CSD,H+ CSD~H: GD: C,D,H+ C,D: C3D2H+ CNDH+ CNH:

71 70 70 69 42 41 29 28

90-100 O-10 90-100 O-10 65 35 70 30

(%)

[ 161 who concluded that two CSHz ’ ions are formed in aniline at 70eV: a cyclic form and a linear form. However, the lower energy metastable formation of C,H,+ ’ results in the predominant formation of the cyclic ion. Our previous low energy 20eV conditions may reflect a larger component of the metastable cyclic product ion, which presumably forms as C,D,H: ’ in the aniline-d,, while the present 70 eV conditions are dominated by the formation of the more energetically expensive linear ion which forms as CSD, H+ ’ . Secondly, it is also interesting that as much as 35% of the C,H: formed contains a hydrogen atom originally found on the amine group and 70% of the CNH: contains a hydrogen atom originally attached to the ring. This indicates that the molecular ion undergoes some rearrangement before fragmentation under 70 eV electron beam conditions. One possibility is the formation of an azepinium ion, a seven-membered ring with the nitrogen atom incorporated, from which the ions C3D2H+ and CNHD+ can be formed readily by simple bond cleavage without further rearrangement (see Fig. 2). 266nm A4PI Figure 3 shows the 266 nm MPI spectra of aniline, aniline 15Nand aniline-d, at a laser pulse energy of 2 mJ. A comparison with the 266 nm spectrum in Fig. 1 shows that at this higher laser pulse energy quite large intensities of additional small-mass ions such as C+ ’ , Cc ’ , C: ‘, C, H+ and CNHT are formed and the molecular ion is now less intense. The aniline-“N spectrum shows clearly the shift of m/z 28 to m/z 29 for the identification of CNH:, and the aniline-2,3,4,5,6-d, spectrum shows that a large fraction of CNH: is CNHD+ . At this higher pulse energy the peaks are slightly broadened. This is probably due to increased coulombic repulsions created by the larger intensity of ions.

176

-

CNDH'

+

C,D,H'

C3D,H+

+

C,ND,H'

D

D

Fig. 2. Decomposition

of deuterated azepinium to form CNDH+ and C3D2Hf.

Nitrogen-containing ions

The nitrogen-containing ion distribution is similar to that observed for the electron beam case, with the percentage of nitrogen-containing fragment ions being found to remain at approximately 20% throughout the range of laser pulse energies used. Laser pulse energy dependence of ions

A plot of the ion intensities of the 12 most intense ions as a function of laser pulse energy is shown in Fig. 4. At low laser pulse energies a large percentage of the ionization is detected as higher mass ions, in particular, the molecular ion and C, H6+’ . As the laser pulse energy is increased the contribution of these to the % TIC decreases and that of several smaller ions (C’ ‘, Cl., C,+’ and C,H+) increases. An especially large increase is observed in the % TIC owing to the C, H+ ion which doubles over the recorded range and becomes the most intense ion in the spectrum at high laser pulse energies. The % TIC for all ions becomes approximately constant at higher laser pulse energies. The observed lower laser pulse energy behavior, the % TIC for larger mass ions decreasing as that for the smaller mass ions increases, is in keeping with class A behavior or an ionic ladder-switching mechanism in which the molecular ion is formed initially but fragments at higher laser pulse energies owing to the absorption of additional photons. This is exactly what would be expected for the resonant MPI case. The plateau of ion intensities observed at higher laser pulse energies is probably due to the transitions involving the excitation of the various

177

a)

m/z

10

20

30

40

50

60

70

60

90

m/z

10

20

30

40

50

60

70

80

90

c)

,

m/z

1;

2’0

3b

C3DC

40

5b

C6D6NH2f

6b

7b

6b

Fig. 3. 266nm MPI of (a) aniline, (b) aniline-“N, energy of 2.0 mJ.

90

and (c) aniline-2,3,4,5,6-d,

at a laser pulse

fragment ions becoming saturated so that further increases in laser intensity have no effect on the fragment ion distribution. The sustained presence of a significant amount of the molecular ion and C,H,+’ (another high mass ion) at high laser pulse energies leads to an interesting question. Why do some aniline molecules absorb a large number of photons to produce energetically expensive fragments such as C+',C: ’ etc., while under the same laser pulse energy conditions some molecules only absorb a small number of photons and remain at the molecular ion level? This question has been addressed by Baer and Carney [17] who also reported intense C+’ fragment ions and intense molecular ions for resonant MPI at 460nm in aniline. They suggested that not all the aniline molecules ionize after enough photons have been absorbed to exceed the ionization energy, but that an additional photon is absorbed to some autoionizing state.

0.00

,,,,~~,,,,,,,,,,,,“,““““‘I”““ml’n.”’,,,,,”,,,: 0 40 0.00

2.00

~ose~~~;ulse’%.rg; '&nJ)

0 00

L

0.00

Fig. 4. Percentage total ion current vs. laser pulse energy for the most intense ions at 266nm MPI: (a) the higher intensity ions; (b) the lower intensity ions.

The aniline ion formed by the autoionization may be highly vibrationally excited and further photon absorption might be restricted because of poor Frank-Condon factors, leaving the molecular ion trapped. Deuterated

ions

Table 2 identifies the deuterated ions formed at 0.25m.I per pulse. Data obtained at higher pulse energies indicate that the fractional contribution of the deuterated ions does not change as the laser pulse energy is increased. CSH6+’ is again found to be predominantly C,DSH+ ’ . This is similar to the 70eV electron beam data, but contrary to some of our previously published 266nm MPI data [IS] which showed that CSDSH+’ and CSD4H:’ are formed

179 TABLE

2

Identification

of deuterated

ions at 266 nm and 0.25 mJ per pulse

ml2

% TIC

Ion(s) in aniline-d,

mlz

Aniline ion (%)

CSH,C’ Cj NH: C, NH: C,NH: C,NH:’

66 54 52 42 41

6.1 2.9 2.7 3.6 4.2

CSDSH+’ C3ND,H; CjNDH+ CzND3H+ CzND,H+’ Cz NDH;

C, H:

39

4.6

C,H:’

38

4.3

C,H’

37

4.9

CNH:

28

4.4

GD: CjD2H+ C,DH; CjD;’ C,DH+’ C3D+ C3H+ CNDH+ CNH;

GH:

27

2.5

C2H’

25

0.8

71 56 53 44 43 42 42 41 40 40 39 38 37 29 28 30 29 28 26

Approx. 100 Approx. 100 Approx. 100 90-100 30-60 10-70 30-60 40-90 O-30 40-80 20-60 90 10 65-100 o-35 O-90 35-100 O-50 100

Ion in aniline

GD: C,D2H+ Cz DH; C2D+

in an approximate ratio of 3: 1 when using unfocused 266 nm light in which only three 266 nm photons were absorbed. However, a focused beam was used to obtain these data and the C,(D,H)z’ ion is probably formed by a more energetic four-photon process producing mainly C5D5H+‘, as explained for the electron beam case. It is also apparent that a number of ions formed in the aniline-d, spectrum, for example, C,ND, H+ , CZND, H+ ’ , C3D, H+ , C3DH+‘, C3H+, CNDH+ and C,D,H+ are necessarily the products of either molecular or ionic rearrangement or scrambling. These ions could all be formed relatively simply from the azepinium ion structure shown in Fig. 2. Ionic ladder-switching

The data in Table 2 can be used to consider the possibility of ionic ladderswitching in the formation of smaller mass ions. For example, the occurrence of a postulated decomposition such as C,H:. losing H to form C3H+ can be examined by comparing the relative abundances in the deuterated counterpart of each ion. A statistical decomposition of C,D2f’ (40-80% of all C,(D,H):‘type ions) would produce only CD’, and a statistical decomposition of C3DH+’ (20-60%) would produce 50% C,D+ and 50% C3H+. These decompositions would produce overall percentages of 50-90% C,D+ and lo-30% C3H+. The experimentally observed abundances of C3D+ (90% TIC) and

180

C3H+ (10% TIC) can therefore be accounted for by statistical ionic decomposition. A similar argument reveals that the majority of the non-nitrogencontaining ions with m/z < 39 could be precursors to other small mass ions and C2NH4+ could statistically decompose to give C,NHc’ + H’. The deuterated counterparts of C,NH: and C3NH2+ are detected as approximately 100% C,ND2H: and C3NDH+ respectively. This is interesting because the observation of 100% C,ND,Hl implies a straight cleavage of the molecular ion without rearrangement. Also, if C3NDH+ is formed from C,ND,H$ this indicates a specific loss of HD and not a statistical loss of (H,D),. However, this would still be in keeping with an ionic decomposition mechanism. Because more than one ionic pathway may contribute to an ion’s overall intensity this discussion does not indicate that ionic ladder-switching is definitely taking place, but we can conclude that the data are consistent with an ionic ladder-switching mechanism. 532 nm MPI Figure 5 shows the 532 nm spectra of aniline, aniline-i5N and aniline-d, at a laser pulse energy of 5OmJ. A comparison with the low laser pulse energy spectrum shown in Fig. 1 shows that at this higher laser pulse energy the lower mass ions are considerably more important, with C+’ , CT’, C,H2+’ and CNH: being the most intense peaks in the spectrum. The molecular ion is barely detectable in all spectra. The “N spectrum shows that most of the m/z 28 intensity is due to CNH: , a considerable amount of the m/z 38 intensity is due to C2N+ and all the m/z 52 intensity is due to C,NH: . The aniline-d, spectrum shows a significant amount of CNH: appearing as CNHD+ and all the C,NH,f appearing as C3NDHf. Nitrogen-containing ions

In comparison with electron beam and 266 nm MPI the nitrogen-containing ions constitute a much larger percentage of the total ion current. At lower laser pulse energies, 51% of the ions formed contain a nitrogen atom; at higher pulse energies this figure becomes 32%. Laser pulse energy dependence

A plot of the ion intensities for the 12 most intense ions as a function of laser pulse energy is shown in Fig. 6. As the laser pulse energy is increased the % TIC curves of several of the smaller mass ions (C’ ‘, C,+’ , C,H: ‘, C,H+ ) increase and start to plateau at the higher laser pulse energies. Simultaneously several of the larger mass ions (CNH,+ , C3H,+ , C, Hz ’ , Cz N+ , C3NH: ) show a decrease in % TIC as the laser pulse energy is increased, but these curves also tend to plateau under the more intense photon flux.

181 a)

C?

C2f

CNH2’

!

mfz IO

I 20

30

40

50

60

70

80

90

b)

C6H5N15H 2:

m/z 10

20

30

40

50

60

70

80

90

CNHD+

5NH2:

Fig. 5. 532 nm MPI spectra of (a) aniline, (b) aniline-“N, per pulse.

and (c) aniline-2,3,4,5,6-d,

at SOti

182

Fig. 6. Percentage total ion current vs. laser pulse energy for the most intense ions at 532nm MPI: (a) the higher intensity ions; (b) the lower intensity ions.

The laser pulse energy dependence seems to indicate mixed behavior similar to that observed for 266nm MPI; class A at lower laser pulse energies, and class C at higher laser pulse energies. However, at this wavelength the molecular ion is virtually unobservable at the lowest laser pulse energies used, which is not consistent with a class A mechanism. Since two 532 nm photons populate the same excited state as one 266 nm photon we might have expected some resonant enhancement in the overall 532 nm (2 + 2) ionization process. This is clearly not the case. The 532 nm and 266 nm MPI spectra are markedly different. Most noticeably the 532 nm spectra show no molecular ion and no C,Hl’ ion. Another possible explanation for this comes from a consideration of what

183

might happen after absorption of the third 532 nm photon. There have been at least three reports in the literature indicating that rapid neutral dissociation may occur when the aniline molecule is excited to an energy level equivalent to the absorption of approximately three 532 nm photons [ 18-201. Dietz et al. [18] using 293.9 nm observed ionization via a resonant 1 + 1 process, but in a two-color experiment in which a 532 nm beam overlapped the UV beam, they found a marked decrease in ionization. They concluded that the upper state formed in the overlap experiment (one 293.9 nm photon plus one 532nm photon) was predissociative at a far higher rate than the subsequent up-pumping rate. The presence of a state or states in this energy range, which lead to fast dissociation of the neutral molecule, is supported by experimental work carried out by Rettner and Brophy who reported MPI data on aniline using 294nm, but were unable to ionize the molecule at 532nm [19]. Further support comes from MPI and photoacoustic data reported by Moll et al. [20] who carried out a two-color experiment involving a pump laser pulse tuned to the Sr O-Otransition and a 532 nm transfer pulse with a variable time delay between the two pulses. When the time delay between the two pulses was small (on the nanosecond timescale) they noted a decrease in ionization as Dietz et al. had done. In addition to the decreased ionization signal they also noted an increased photoacoustic signal which they attributed to the formation of neutral dissociation products. Moll et al. pointed out that the lowest bond energy in the molecule, that of the N-H bond is only 3.8eV. However, it is quite apparent that the energy of three 532 nm photons (6.99 eV) is more than enough to break any of the bonds in the molecule. Furthermore, in a focused laser beam experiment where multiple photons can be absorbed from the same laser pulse, the breaking of two or possibly more bonds to produce neutral fragments from the atoms originally contained within the ring of the molecule need not be ruled out from an energetic standpoint. We therefore propose that the mechanism for MPI in aniline at 532nm involves a first step dissociation of the molecule into neutral polyatomic fragments, which then absorb further photons from the laser pulse and are ionized. This explains the apparent class A behavior at low laser pulse energies; it is the class A behavior of neutrals formed in a class B first step. It is not clear what the polyatomic neutral fragments formed in the first step are, but from the spectrum shown in Fig. l(b), obtained at the lowest laser pulse energies possible, C,NH;, C,H; and C3H; would all seem to be reasonable possibilities. A simple cleavage of the neutral aniline molecule, C,H,NH,, could lead to C, Hi and C,NH; or C, Hi and C, NH,. On the laser pulse energy plots the ionized counterparts of two of these neutral molecules, C,NHz and C3Hc have intensities which decrease at lower pulse energies while the intensities of smaller mass ions increase. We propose then that the MPI mechanism

184 TABLE

3

Identification Ion in aniline

of deuterated

ions at 532nm

mlz

% TIC

63 54 52 42 41

1.3 1.9 7.7 1.4 1.5

C,H:’ CxH+

39 38 31

9.4 6.7 5.8

CNH;

28

18.5

C*H:’

26

3.4

C, NH: C,NH; C2NH: CzNH:

GH:

and 5.0mJ per pulse

Ion(s) in aniline-d,

mlz

Aniline ion (%)

CO:

66 66 53 44 43 42 42 40 38 37 29 28 28 21

100 100 100 100 60 40 100 100 80 20 50-75 25-50 85 15

C3 ND2 H; CJNDH+ Cz ND, H: CzNDzH+’ C2 NDH; ’ GD: C,D;’ C,D+ C,H+ CNDH+ CNH; C2D;’ C*DH+’

at 532 nm is class B (neutral fragmentation) the neutrals formed in the first step.

followed by class A ionization of

Deuterated ions and ionic ladder-switching

Table 3 identifies the deuterated ions in the aniline-d, spectrum at the relatively low laser pulse energy of 5.0 mJ per pulse. These deuterated data show less rearrangement than that obtained at 266nm. The deuterated contributions for several ions, for example C5HT (100% C,D: ), C3NH: (100% C,ND,H:), C,H,f (100% C,Dc) and C,H:’ (100% CD,+‘) are indicative of a direct cleavage of the neutral molecule or molecular ion. This is in keeping with our proposed initial rapid dissociation of the neutral molecule. Other ions, for example CNDH+ , C3 H+ , C2DH+ ’ , appear to indicate a rearranged precursor molecule or molecular ion. However, (as we discussed for the 266 nm data) just as C,NH: , which is observed as 100% C3NHD+, could be produced by a specific HD elimination from the C,NH,D: ion, CNDH+ , C3H+ and C2DH+’ could all be formed by fragmentation of CNHD+. This secondary ionic ladder-switching, class A type mechanism, would also be in keeping with the laser pulse energy dependence data and our proposed mechanism if the initially formed neutral fragments included the C,NH, species, which appears specifically as C, NH, D, in the aniline-d,.

Figure 7 shows the 355 nm spectra of aniline, aniline-“N and aniline-d, at a laser pulse energy of 6.0 mJ per pulse. Ionization was detected at laser pulse

185

a)

CNH2+

c$++

Ll 1

c)

40

C&NH2:

lb

1

50

80

70

80

90

50

80

70

80

90

C?

I-

C2

CNHD+

C$

j

m/z IO

20

.o

Fig. 7. 355 nm MPI spectra of (a) aniline, (b) aniline- “N, and (c) aniline-2,3,4,5,6-d, per pulse.

at 6.Om.l

186

energies significantly lower than those needed at 532 nm. This is probably due to the increased energy of each 335 nm photon (3.5 eV at 355 nm as compared with 2.3 eV for 532 nm). Even though a much lower photon flux is used, the 355nm and 532nm spectra appear to be quite similar at first inspection. However, close examination reveals a number of differences. Nitrogen-containing ions

Over the range of laser pulse energies studied (0.50-6.00mJ) a constant 25% of the TIC is composed of nitrogen-containing ions. At 532nm the % TIC for nitrogen-containing ions was found to vary from 52% at high laser pulse energies to 32% at low laser pulse energies. This indicates a greater percentage of ionization from the aniline ring at 355 nm MPI when compared with 532nm MPI. Laser pulse energy dependence

Figure 8 shows the laser pulse energy dependence for the 12 most intense ions observed at 355 nm. In general terms this plot is similar to that obtained at 532 nm, with lower mass ions increasing in intensity at the expense of higher mass ions as the laser pulse energy is increased. This shift of ion intensity to lower mass ions can be traced primarily to a decrease in the % TIC of ions in the m/z range 36-42. The most important of these is the C,H: ion which decreases from 27.9% to 7.4% of the TIC as the laser pulse energy is increased from 0.5 to 6.0 mJ per pulse. Given the similarity in appearance of the spectra and the laser pulse energy dependence plots at the two wavelengths, and taking into consideration that absorption of two 355 nm photons takes the molecule to the same energy as the absorption of three 532nm photons, it seems likely that dissociation of the neutral molecule is the tirst step in the overall ionization mechanism for the majority of the molecules. However, the presence of a molecular ion which, although small, is larger at all energies than the almost non-existent molecular ion observed at 532 nm, indicates that there must be another parallel but minor mechanism which involves formation of the molecular ion. We propose therefore that the majority of the neutral aniline molecules decompose into smaller neutrals (as described for the 532 nm MPI) which are then ionized and exhibit class A behavior as indicated by the laser pulse energy dependence. A small fraction of the molecules are ionized to form the molecular ion which subsequently photofragments. Deuterated ions and ionic ladder-switching

Table 4 identifies the deuterated ions in the aniline-d, spectrum at 6.0mJ per pulse which is the high energy end of the range of laser pulse energies used. Data obtained at the low energy end (0.5 mJ per pulse) are almost identical in the breakdown for contributions of different deuterated ions to a particular

187

53) 30.00 -

2 ?! : u

20.00

.-E 0

5

10.00

-

R

0.00

m

0.00

1 .oo

4.00

2.00

laser

pulse

energy

5.00

6.00

7

(mJ)

b) 6.00

.

.

.

0.00

m

0.00

I

1

.oo

2.00

laser

3.00

pulse

4.00

energy

5.00

6.00

7

(mJ)

Fig. 8. Percentage total ion current vs. laser pulse energy for the most intense ions at 355 nm MPI: (a) the higher intensity ions; (b) the lower intensity ions.

ion, with the exception of C2H: * and CH+ which at the lower energy appear as 100% C,D2f’ and 100% CD+ respectively. When the data are compared with the 532nm deuterated data, one of the most noticeable differences is contained in the composition of C, Hc . In the aniline-d, spectrum at 532 nm, 100% of C,H,f is formed as C,D,+ , but in the 355 nm spectrum only 65% is formed as C, H3+, with approximately 35% being formed as C,D,H+ and a very small amount being formed as C3DH:. (A slightly larger amount of CjDH2+ is formed at the lower pulse energies.) Since a straight cleavage reaction of the neutral aniline-d, molecule would lead to 100% C,D:, it is reasonable to assume that these mixed ions (C, D2 H+ and C,DHl ) are formed directly from the molecular ion or aniline molecule because no other

188 TABLE

4

Identification

of deuterated

Ion in aniline

ions at 355 nm and 6.0mJ per pulse

ml2

% TIC

Ion(s) in aniline-d,

mlz

Aniline ion (%)

52 39

2.2 7.4

C,NDH+

GH:

C,H;’

38

1.4

C,H+

37

0.9

CNH;

28

12.1

GH:

27

3.5

C*H;’

26

5.9

CzH+

25

6.2

CH+

13

3.0

53 42 41 40 40 39 38 37 30 29 28 30 29 28 27 26 25 26 25

100 65 35 Very small 75 25 85 15 O-10 50-80 1O-40 O-40 60-100 70 30 80 20 70 30

C,NH:

GD: C3D2H+ C,DH: C,D;’ C3DH+’ C3D+ C,H+ CND; CNDH+ CNH: CID: C2D2H+ C2D:’ CzDH+’ CzD+ C,H+ CD+ CH+

possible large mass ionic precursors to the C3H: ion are observed in the 355 nm spectrum. Under collision-activated dissociation (CAD) conditions it was found that in addition to the molecular ion, the ionic sources of C,Hl are C,H.L C,H: ’ and C,H: [21], none of which are found to any extent in the 355 nm spectrum. The most likely route to the rearranged C,H: is therefore via a molecular ion having enough energy above the ionization energy so that atom rearrangement can take place in competition with direct cleavage. Observation of a small but significant molecular ion is supportive of this proposed mechanism. The deuterated data indicate that quite extensive ionic ladder-switching may be taking place at 355 nm. The formation of CNDH+ from C3NDH+ is a distinct possibility, C3NDH+ being the only intense large mass nitrogencontaining ion (apart from the molecular ion) observed in the spectrum. Bearing in mind that the values for deuterated ions may have quite large uncertainties (see Experimental section) once C,D: is formed (via the presumed direct cleavage of the neutral molecule) a possible ionic ladder could be C,D: -C3D;-

-+C,D+

or

C,D: + C2D:’ -+ C2D+ + CD+

Because the larger part of C,H: appears as C2D2H+ (60-100%) and only the

189

smaller part as C,D: (O-40%) it appears that the majority of C2H: formation is not included in this ionic ladder and must be formed via a different mechanism involving the molecular ion. C,D,H+ must also be formed via a different mechanism from C,D,+ as already discussed, but once formed a possible ionic ladder is C3D2H+’ + C3D:‘/C3DH+’

+ C3D+/C3H+

or C3D2H+ --+CzDH+‘/C,D,f’ SUMMARY

+ C2D+/C2H+ + CD+/CH+.

AND CONCLUSIONS

In comparing the fragmentation patterns produced at the three MPI wavelengths it is found that 266nm ionization is closest to that produced by electron beam. The 532 nm and 355 nm ionizations are both very different from the 266 nm and 70 eV ionizations but bear some close similarities to each other. Both 70 eV and 266 nm MPI produce a similar distribution of ionic fragments, with the low energy 266nm spectrum being practically indistinguishable from the 70eV spectrum. If it is assumed that the ionic decomposition produced by the electron beam proceeds by a statistical, quasi-equilibrium theory mechanism [22] then it is reasonable to assume that the low energy 266nm MPI proceeds by a similar mechanism, that is, a class A type decomposition of the molecular ion energized by the absorption of a small number of photons. This is supported by the laser pulse energy plots, which show decreasing intensities of the larger ions (including M+’ and C,H,+‘) with concurrent increasing intensities of smaller ions. It is further supported by the deuterated data which show considerable ionic rearrangement and that both rearranged fragment ions and non-rearranged fragment ions can undergo ionic ladder-switching, Only when the laser pulse energy is increased do the 70 eV and 266 nm MPI begin to differ, with 266 nm MPI producing spectra in which small mass ions become increasingly abundant. The observed 532 nm MPI fragmentation pattern is quite different from the 266nm MPI pattern. The most obvious differences are the almost total lack of the molecular ion, the non-existence of the C,H,+’ ion and the most intense ions being the low mass ions even when using the lowest laser powers possible to produce ionization. These observations plus the laser pulse energy dependence of the fragment ion intensities are consistent with a first-step neutral dissociation (class B) followed by class A behavior of the ionized neutral fragments which are proposed to be C,NH, and C,H; or C,NH; and C,H;. The initial class B dissociation is supported by the deuterated data which

190

indicate that several ions are formed by direct cleavage of the molecule. The deuterated data also support the subsequent class A mechanism, indicating ionic ladder-switching for fragments such as C3NH: and C3H: . The 355 nm MPI fragmentation pattern is very similar to that observed at 532 nm. The laser pulse energy dependence plots and the deuterated data are again consistent with a first-step neutral dissociation (class B) followed by class A behavior of the ionized neutrals. However, at this wavelength, the deuterated data indicate that this is not the only mechanism followed and that a small fraction of the molecules undergoes an initial class A type ionization which involves rearrangement of the atoms. This is further supported by the presence of a small molecular ion at all energies. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

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