J . Phys. Chem. 1987, 91, 1114-1120
1114
plateaus do not correspond to the same [Hg*]. The plateau for Hg(3Pz) formation results, in part, because of a loss process that depends on [Hg], i.e., the conversion of Hg(3P2) and Hg(3Po) to Hg,* by three-body recombination. Hg* Hg Ar Hg2* + Ar (All
+
Hg,*
- + +
2Hg
hv (335 nm)
The excited-state mercury dimer Hg2* can be detected by observation of the 335-nm emission. The dependence of this emission intensity upon [Hg] and Ar pressure is shown in Figure 13; the Hg2* formation is second order in Hg since in this concentration range [Hg*] is proportional to [Hg]. In fact, this flowing-afterglow mercury metastable reactor should be a good way to study the three-body recombination of the Hg(3P2)and Hg(3Po)atoms in He, Ne, and Ar buffer gases; however, such work was not attempted for this study. This flowing-afterglow technique might also permit isolation and characterization of long-lived Hg2* states by doing experiments at longer times in the flow reactor. The quenching of the [Hg(3P2)] can lead to formation of Hg(3PI). In order to accurately determine the Hg(3P1) formation rate, the radiation trapping of the Hg(3PI-1So) transition at 253.7 nm was characterized as a function of [Hg]. This was accomplished by monitoring the Hg(3PI-1So) and the NO(A-X) relative emission intensities from the reaction with NO Hg(3P2)
+ NO
-
-
-
NO(A,v'=O)
+ Hg
AHo,=
[Hg] was (0.8-1.6) X 1013 atom cm-3 and a correction to the observed Z(Hg?P,) for the degree of radiation imprisonment was made according to the results shown in Figure 14. The vibrational relaxatio# of the HgX(B) formed in high vibrational levels by chemical reaction was a subject of interest. For that reason the operation of the metastable mercury atom flowing-afterglow was characterized for a broad range of He, Ne, and Ar pressure. The results are summarized in Figure 15. As the pressure is increased, the discharge voltage must be decreased for optimal Hg(3P,) production. Experiments were done up to 25 Torr for the three carrier gases; even higher pressures could be used if a larger pump was utilized to reduce the flow time between the discharge and observation zone. As illustrated in Figure 12, higher [Hg] is required for N e and H e carrier gases than with Ar for optimum [Hg,3P2]. This is probably a consequence of faster diffusion of Hg(3P2)to the walls followed by quenching in the lighter gases. Registry No. ICI, 7790-99-0; IBr, 7789-33-5; CCI,, 56-23-5; CCI,F, 75-69-4; CHC13, 67-66-3; CH2C12, 75-09-2; CHjCI, 74-87-3; COCI2, 75-44-5; CCI,Br, 75-62-7; CBr,, 558-13-4; CF2CIBr, 353-59-3; CF2Br2, 75-61-6; CHBr3,75-25-2; ICF2CF2Br,421-70-5; PCI,, 7719-12-2; SCI,, 10545-99-0; S2CI2,10025-67-9; PBr,, 7789-60-8; F,CNCI, 28245-33-2; F3CNC12, 13880-73-4; CIFZCNCIF, 33757-11-8; CIF2CNC12, 2824534-3; F3CNC1Br,88453-17-2; ICN, 506-78-5; BrCN, 506-68-3; CF,NCIBr, 88453-17-2; CF3NCI2, 13880-73-4; CF2CINCI2,28245-34-3; CF2ClNFCl, 33757-1 1-8; CF2NC1, 28245-33-2; CI,, 7782-50-5; Br,, 7726-95-6; I,, 7553-56-2; Hg, 7439-97-6.
0.5 kcal mol-' (A2)
Hg(3PI) + N O
AHo, = -13 kcal mol-'
other channels
The observed Z(Hg(3Pl))/Z(N0,A) ratio,f, as a function of [Hg] is shown in Figure 14. The increase of the ratio with declining [Hg] is a consequence of diminished radiative imprisonment. For the intramultiplet relaxation studies reported in the main paper,
(53) Oba,D.; Zhang, F. M.; Setser, D. W. J. Chem. Phys., to be submitted for publication. (54) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the pblock elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., I11 3 and 13.)
-
Molecular Beam Photodlssociatlon Study of Methyl Nitrite In the Near-Ultraviolet Region Beat A. Keller, Peter Felder, and J. Robert Huber* Physikalisch Chemisches Znstitut der Uniuersitat Zurich, CH-8057 Zurich, Switzerland (Received: August 25, 1986)
The photodissociation of methyl nitrite (CH30NO) at 350, 248, and 193 nm was studied by photofragment translational spectroscopy in a pulsed molecular beam. Translational energy distributions P(EJ in the center-of-mass (CM) frame were derived from the timeof-flight spectra of the neutral photofragments CH30 and NO. Angular distributionsof the photoproducts were determined by measuring the dissociation yield as a function of the laser polarization angle, providing anisotropy parameters p(350 nm) = -0.70 & 0.05 and p(248 nm) = +1.4 & 0.1. These strongly anisotropic distributions imply that the dissociation process occurs on a subpicosecond time scale. Photodissociation at 350 nm is discussed in conjunction with a LIF study of the NO internal state distributions. At 248 nm the transition moment is almost parallel to the fragment recoil direction and dissociation is thought to occur on a strongly repulsive upper-state potential energy surface. At 193 nm the average translational energy amounts to 43 h 1% of the available energy. This fraction is significantly smaller than the 53 h 2% found at 248 nm.
Introduction The investigation of phot&issociation processes of plyatomic molecules has become a profitable way to elucidate the microscopic dynamics of chemical reactions.~,2 A~~~~ the various experimental approaches, probing of the product states by laser-indud fluorescence (LIF)on the one hand and molecular beam photofragment translational spectroscopy on the other hand have been
particularly fruitful. These two methods differ in their advantages and disadvantages and can be considered complementary. LIF spectroscopy in combination with Doppler spectroscopy, fully exploiting various polarization schemes, has in favorable cases been capable of yielding an almost complete microscopic characterization of dissociation p r o c e ~ s e s . ~ -In~ contrast, photofragment SPectroscoPY with maSS spectrometric product detection may not
(1) Jackson, W. H.; Okabe, H. In Aduances in Photochemistry, Vol. 13, Volman, D. H., Gollnick, K., Hammond, G. S.,a s . ; Wiley: New York, 1986.
(3) Vasudev, R.; a r e , R.N.; Dixon, R. N. J. Chem. Phys. 1984,80,4863. (4) Dubs, M.;Brtihlmann, U.; Huber,J. R. J. Chem. Phys. 1986,84,3106. ( 5 ) Qian, C. X.W.; Noble, M.; Nadler, I.; Reisler, H.; Wittig, C. J . Chem. Phys. 1985,83, 5573.
(2) Simons, J. P. J. Phys. Chem. 1984, 88, 1287.
0022-3654/87/2091-1114$01.50/00 1987 American Chemical Society
Photodissociation of Methyl Nitrite be able to provide such fine details, but it has the advantage of not being restricted to small (mostly diatomic) photofragments with accurately known spectroscopic properties. Furthermore, time-of-flight (TOF) spectra and angular distributions of photofragments allow the extraction of translational energy distributions and anisotropy parameters in a straightforward way.6 Nitrous acid (HONO) and its organic esters (RONO) have been the subject of photochemical investigation for a long time.’ Several detailed studies of methyl nitrite,8-’0ethyl nitrite,’&” and tert-butyl nitriteI2 were performed by different groups. As in nitrous acid, the near-UV absorption spectra of simple organic nitrites consist of a structured band system in the 300-380-nm range (SI,n T*)and a broad unstructured band centered near 210 nm (S2,T T*,and possibly higher excited states). In all cases the dominant photofragmentation pathway leads to N O and alkoxy radicals RO. Lahmani and c o - ~ o r k e r shave ~ ~ investigated the photodissociation of SI methyl nitrite by probing the N O photofragment with two-photon LIF spectroscopy. These authors derived rotational and vibrational distributions at several dissociation wavelengths between 364 and 3 18 nm. They concluded that the excess energy deposited in the N-0 stretching mode of SI methyl nitrite is retained in the N = O moiety and appears as vibrational energy in the free N O photoproduct. We subsequently reported preliminary results from photofragment translational spectroscopy of methyl nitrite at 350 and 248 nm.I3 At a dissociation wavelength of 248 nm, the photofragments recoil with an average translational energy of 53 f 2% of the available energy, in excellent agreement with the prediction of a simple impulsive model. Quite surprisingly, at 350 nm as much as 57 f 4% of the available energy is channelled into translation. Taking into account the NO vibrational and rotational energy determined by Lahmani et al. one concludes that the methoxy fragment is left with only little vibrational energy. In the present communication we report on further experiments aimed at getting a more complete picture of methyl nitrite photodissociation. TOF spectra at a dissociation wavelength of 193 nm were measured in order to probe the high-energy side of the second UV absorption band. The associated translational energy distribution is compared to the one on the low-energy side at 248 nm. Furthermore, the translational energy distribution at 350 nm is discussed in detail by taking into account the vibrational distribution of the emergent N O product. Anisotropy parameters derived from photofragment angular distributions are used to extract information on the symmetry and the lifetime of the SI and S2 states. In the concluding section we review the main experimental results obtained so far on the photodissociation of methyl nitrite and of the related molecules H O N O and dimethylnitrosamine.
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1115
--
Experimental Section I . Apparatus. The experimental set-up has been briefly presented in ref 13 and a more complete description will be given here (see also ref 14). Our beam apparatus was built in close accordance to Wilson’~’~ original design except for the use of a pulsed beam source. The directions of the laser, molecular beam, and fragment detection are fixed to a mutually perpendicular configuration allowing a very compact construction (see Figure (6) Bersohn, R. IEEE J . Quantum Electronics 1980, Q E l 6 , 1208 and references quoted therein. (7) Calvert, J. G.; Pitts Jr., J. N . Photochemistry;Wiley: New York, 1966. (8) Lahmani, F.; Lardeux, C.; Solgadi, D. Chem. Phys. Lett. 1983, 102, 523. (9) Benoist d‘Azy, 0.; Lahmani, F.; Lardeux, C.; Solgadi, D. Chem. Phys. 1985, 94, 247. (10) Ebata, T.; Yanagishita, H.; Obi, K.; Tanaka, I. Chem. Phys. 1982, 69, 27. (1 1) Tuck, A. F. J . Chem. SOC.,Faraday Trans. 2 1977, 73,689. (1 2) Schwartz-Lavi, D.; Bar, I.; Rosenwaks, S. Chem. Phys. Lett. 1986, 128, 123. (13) Keller, B. A.; Felder, P.; Huber, J. R. Chem. Phys. Lett. 1986, 124, 135. (14) Keller, B. A. Ph.D. Thesis, Universitat Ziirich, 1986. (15) Busch, G. E.; Cornelius, K. E.; Mahoney, R. T.; Morse, R. I.; Schlosser, D.; Wilson, K. R. Rev. Sci. Instrum. 1970, 41, 1066.
%PI cm
Figure 1. Photodissociation apparatus (vertical section). Numbered components are (1) pulsed valve, (2) beam interaction. region, (3) cold surface, (4) axial ionizer, (5) quadrupole mass filter, (6) electron multiplier, (7) detector gate valve. FIGl and FIG2 denote locations of fast ionization gauges. Large arrows: A, B, C = oil diffusion pumps, D = turbomolecular pump.
1
I
I
I
1
1
1
t(P) Figure 2. Density vs. time profiles of a neat CH,ONO beam measured with fast ionization gauges. Triangles and circles are data points measured at locations FIGl and FIG2, using a dwell time of I O ps and a mutual distance of 11.1 cm. The solid line was calculated by convoluting the signal at the first gauge with a velocity distribution characterized by B = 700 ms-’, S = 5 (see eq 1 in the text). 200
400
600
1). The pulsed molecular beam was generated by expanding the gas through the 0.5-mm-diameter nozzle of a home-built Gentry-Giese-typeI6valve. The conical exit section of the valve was chosen to be relatively flat in order to avoid excessive cluster formation in the neat methyl nitrite beam. Typically, a source pressure of 1 bar C H 3 0 N 0 yields pulses of about 100 ps duration (fwhm) at a repetition rate of =3-4 Hz. A constant reservoir pressure during data collection was achieved by a microproces(16) Gentry, W. R.; Giese, C. F. Rev.Sci. Instrum. 1978, 49, 595.
1116
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Keller et al.
sor-controlled pressure regulating system. The molecular beam was collimated by two skimmers (1.5 mm diameter, Beam Dynamics, MN) mounted between the differential pumping stages of the vacuum chamber. Two fast ionization gauges (FIG's)I6 were used to determine the velocity distribution of the molecular beam. To this end the ion current was fed into a Biomation/Nicolet signal averaging system. The pulse shapes shown in Figure 2 were measured at a distance of 62 and 173 mm from the nozzle. Assuming a beam velocity distribution of the form1'
n(u)
- u2 exp[-(t.
- o)*/cY*]
(1)
the mean velocity O and the width cy of the speed distribution were determined iteratively from the delay and broadening observed to be caused by the 11 1-mm flight path difference between the two FIG'S. The knowledge of 0 and CY is required for the transformation from the laboratory to the center-of-mass (CM) frame (see next section). Calibration experiments with rare gas beams indicated that during operation the source temperature rises to about 325 K due to ineffective cooling of the valve body. In the present experiments with methyl nitrite we found 0 = 700 m s-l and a = 140 m s-l, corresponding to a speed ratio of 5 , a most probable velocity of 727 ms-*, and a translational temperature of 70 K. The photodissociation process takes place in the main chamber, 49 mm in front of the nozzle, where the background pressure is 4X mbar. A small fraction of the resulting fragments enter mbar and are a bakeable UHV chamber evacuated to =3 X detected with a Balzers QMG-5 1 1 quadrupole mass spectrometer equipped with an axial-beam ion source. Ions transmitted through the mass filter impinge on a 17-stage Cu-Be secondary electron multiplier and are converted to TTL pulses by a preamplifierdiscriminator-amplifier combination (Balzers Q R M 101). Time-of-flight (TOF) spectra were measured with a 40-MHz multichannel scaler (800 channels, 1 p s dwell time) interfaced to a PDP 1 1 /23 computer. The average flight length of the neutral photofragments is 12.3 cm. The small delay due to the ion transit time through the quadrupole is calculated for singly charged ions = cymliz( p s ) , where m denotes the ion mass by the formula (amu) and a is an individual constant of the analyzer, determined by pulsing the focusing potential of the ion source. Most of our experiments were performed with CY = 2.75. Photodissociation of the molecules was induced by a Lumonics TE-861 excimer laser operated either at 350 nm (XeF), 248 nm (KrF), or 193 nm (ArF) with pulse energies of 50, 100, and 50 mJ, respectively. The laser beam was focused to a spot of 1 X 3 mmz at the intersection with the molecular beam. Angular distributions of the photofragments were determined at 350 and at 248 nm by measuring the product yield as a function of the laser polarization angle. To this end the laser beam was passed through a Glan-Foucault polarizer (Optics for Research) mounted in a rotation stage. The resulting linearly polarized beam had a pulse energy of 10 and 20 mJ at 350 and 248 nm, respectively. In order to correct for the influence of fluctuations and drift in laser output power, six angular scans (with 500 laser shots per angle) were recorded in alternating directions and subsequently averaged. Methyl nitrite (MEN) was prepared by adding sulfuric acid to a 1:l methanol/sodium nitrite mixture and freezing out the resulting gas at 77 K." Further purification was achieved by vacuum distillation of the product. 2. Data Reduction. Well-known procedure^'^ of coordinate transformation from the laboratory (LAB) to the center-of-mass (CM) frame of the dissociation process were used to extract C M translational energy distributions P(E,) from the T O F spectra. Care must be taken to consider the various experimental param(17) Bernstein, R. B. Chemical Dynamics via Molecular Beam and Laser Techniques: Clarendon: Oxford, 1982. (18) Blatt, A. H. Ed. OrganicSynthesis; Wiley: New York, 1969: Collect. Vol. 11, p 363. (19) Busch, G. E.; Wilson, K. R. J . Chem. Phys. 1972, 56, 3626.
I
,
I
,
,
,
,
,
I
70 90 110 Flight time (#SI Figure 3. Photodissociation of C H 3 0 N 0 at 193 nm: TOF distribution at m / e 30. Time axis is not corrected for ion flight time. Circles denote experimental points; the solid line was calculated from the translational energy distribution shown in Figure 4. 50
eters affecting the transformation. Among these are a distribution of flight lengths caused by the laser spot size and by the effective length of the ionization region. This distribution was simulated by a Gaussian with a fwhm of 0.6 mm. The total translational energy distribution P(E,) in the C M frame was obtained by optimizing a trial distribution until a satisfactory agreement between calculated and experimental TOF spectra was reached. Thereby a grid of 11 effective flight lengths and 15 beam velocities was used. The calculated photofragment spectra are therefore based on a weighted superposition of 165 Newton diagrams for every TOF channel. Note that throughout this work E, will always refer to the total translational energy of both fragments. Applying conservation of linear momentum, we can determine the translational energy of one particular fragment from E,. Angular distributions in the LAB frame were obtained by integrating the TOF spectra measured at various polarization angles. Data were collected in intervals of 30' in the range 0' < 8 < 36O0, where 8 is the angle between laser polarization and photofragment detection axis. All data sets in the range 180' < 8 < 360' were added to the corresponding values in the first two quadrants and then averaged. The angular distribution in the C M frame was assumed to be of the form w(29) = (1 26P2(cos 29))/4n (2)
+
appropriate for an electric dipole transition. The factor Pz(cos 0 ) in eq 2 represents the second-order Legendre polynomial in cos d , where 29 is the angle between the laser polarization and the fragment recoil velocity vector in the C M frame, and p = 2b is the anisotropy parameter. The value of /3 was evaluated by a forward convolution procedure similar to the one used to determine the P(E,) distribution. Again 165 Newton diagrams were used; integrated TOF spectra at every lab angle 8 were calculated and the ensuing angular distribution was compared with the experimental one. The best fitting value of ,6 was determined with a least-squares method.
Results I . Translational Energy Distributions. Photofragment time-of-flight spectra and the resulting translational energy distributions of the reaction C H 3 0 N 0 + hv
-
CH,0(X2Z)
+ NO(X*JI)
(3)
at excitation wavelengths 248 and 350 nm have been published rece11t1y.l~They will be discussed in more detail in the next section. Note that the P(E,)functions were also used in the CM-LAB analysis of the angular distributions. Additional measurements were performed at 193 nm; the TOF spectrum obtained with the mass filter set at m / e 30 is displayed in Figure 3. It is attributed to NO' from NO neutral fragments. TOF spectra were also measured at m/e 29 and m / e 15. These were ascribed to the fragment ions CHOf and CH3+formed upon electron impact ionization of the neutral C H 3 0 radical. No signal
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Photodissociation of Methyl Nitrite
1117
TABLE I: Distribution of the Available Energy among the Product Degrees of Freedom after Photodissociation at X = 193, 248, and 350 nm
CH,O fragment'
NO fragmenta
(Evib)
(Et)
3720OC 7775 f 70 37200d 8965 37200' 11495
(&ib)
(Et)
X = 193 nm 8034 f 170 1228 7180 9263 3906 0 7180 11867 0
6659 6659
X = 248 nm 25 7OOc 6710 f 70 25700d 6190 25700' 7940
850 0
4960 4960
6940 f 180 6400 2700 8200 0
4600 4600
X = 350 nm 4070 f 180
14000' 3930 f 70 14055' 11 9409 3690
20
2300
2060 2060
3820
1700 2130
"All energies in units of cm-'. bThe dissociation energy is 14600 f 300 c ~ r - ' .CTOF ~ ~ experiment this work (see also ref 13). dImpulsive model (soft radical limit), see ref 13. 'Impulsive model (rigid radical limit), see ref 13. fLIF experiment, see ref 9. EModified impulsive model. see ref 13. 100
200
lkllmoll
3w
400
Figure 5. Angular distributions of NO photoproducts at two different wavelengths: integrated TOF spectra vs. lab angle 0. The vertical axis is in arbitrary units; 0 = Oo refers to the E vector being parallel to the
detector axis. Circles are experimental points; the solid line was calculated by using the anisotropy parameters given (see text). TABLE II: Angular Distributions in the CM Frame
dissocn wavelength, nm anisotr parameter @' angle x , deg ~ lifetime 7,c ps C M TOTAL TRANSLATIONAL ENERGY (CM-11
Figure 4. Center-of-masstotal translational energy distributions derived from photofragment TOF spectra at 350, 248, and 193 nm. The energy available after bond rupture is marked by arrows.
was detected at the parent ion mass m / e 31, but in view of the rather high electron energy of 100 eV this is not too surprising (see also the remark in ref 20). The total translational energy distribution P(E,),determined as described in the Experimental Section, was found to have an average energy ( E , ) = 15 810 f 350 cm-' and a fwhm of 5800 cm-I. A comparison of the P(E,) at the three dissociation wavelengths is made in Table I and Figure 4. 2. Angular Distributions. The laboratory angular distributions of the NO photofragment at two excitation wavelengths are shown in Figure 5. No angular scans of the CH30 fragment were taken in view of the low signal to noise,levels at the relevant mass to charge ratios m / e 29 or 15. The error bars are standard deviations of the ensemble averaged at each lab angle 8. Solid lines represent the calculated distributions obtained from the forward convolution method described in the preceding section. It should be emphasized that the data analysis rests on the assumption that in the C M frame the distribtuions w(0) and P(E,) are separable, i.e., that the C M angular distribution can be described with a single parameter p independent of other details like, e.g., the vibrational states of the products. (Even for uncoupled w(0)and P(E,) there is a slight coupling between the TOF distribution and the lab angle 0 induced by the CM-LAB transformation. However, within experimental error limits the shapes of the TOF spectra were found to be independent of 8 ) . The best fitting values of p are listed in Table 11. At 350 nm the sign of p is negative, indicating a perpendicular electronic transition, whereas at 248 nm a parallel (20) Wodtke, A. M.; Hintsa, E. J.; Lee, Y. T. J. Chem. Phys. 1986, 84, 1044.
350 -0.70 f 0.05 90 60.4
248 +1.40 i 0.10 27
