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New apparatus for liquid sampling in electron spectroscopy
Volume
CHEMICAL PHYSICS LETTERS
127, number 2
6 June 1986
NEW APPARATUS FOR LIQUID SAMPLING IN ELECTRON SPECTROSCOPY. He(I) SPECTRA OF THE VAPOURS AND SURFACES OF METHANOL, ETHANOL, l-HEPTANOL AND l-DECANOL Roy BALLARD,
Jimmy JONES,
Derek READ
and Andrew
INCHLEY
School of Chemrcal Saences, Unroersrty of East Anglia, Unruerstty Plain, Norwrch, Norfolk NR4 7TJ. UK Received 13 February 1986
A new apparatus is described for maintaming either a flowing or a stationary liquid surface within an evacuated compartment for the purposes of electron spectroscopy, mass spectroscopy, etc. The liquid is made to flow down a tungsten rod which can be cooled or heated by a saturated slush bath or liquid. The position of the rod is variable so that the liquid surface can be held in the best position and orientation. By varying the rate of flow or by completely stopping the flow of liquid it is possible to examine surfaces at different times after formation and to carry out kinetic studies. The He(I) photoelectron spectra are reported of flowing surfaces of methanol and ethanol at - 78°C and also of the vapours above the flow. Spectra of I-heptanol at 0°C and 1-decanol at 20°C are also given. Contamination of these liquid surfaces with loss of spectral structure occurs in the spectrometer within 30 s to 2 min of formation.
1. Apparatus and experimental
methods
In fig. 1 is shown the apparatus for studying liquid surfaces in vacua; it is mostly made of glass. The basic principles are that (1) the sample liquid is degassed in an upper compartment, (2) the degassed liquid passes through a tap and flows down a tungsten rod to the spectrometer, (3) once past the spectrometer, the liquid is caught and frozen out in a suitably cooled tube. The upper compartment is fitted with a filler tube for the sample liquid, a tube with an anti-splash arrangement connected to a vacuum pump and a bath containing a liquid which serves to bring the sample to the correct temperature. Degassed liquid is allowed to flow into the lower half of the apparatus by means of a tap. When it flows past the tap the sample enters a glass tube passing through a second bath of liquid which either cools or heats both the liquid itself and a tungsten rod. Flowing down the rod, the sample passes close to the entrance of the spectrometer then falls into-a tubular “cold finger” in which it is retained as waste at a sufficiently low temperature to reduce its vapour pressure to a negligible value. The apparatus is attached to the sample compartment of the spectrometer by a glass tube of precise 0 009-2614/86/$0350 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
external diameter which fits into a vacuum flange by means of an O-seal. Such an arrangement has the advantage that it can be rotated so that the liquid surface can always be turned towards the spectrometer even if the flow persists in running down only one side of the rod. The flange is fitted with a variable mounting to enable adjustment of the angle of the rod with respect to the vertical; in practice this adjustment means that the distance between the liquid surface and the spectrometer entrance can be varied at will while the spectrometer is running. In addition the mounting of the He lamp is adjustable so that the radiation can be accurately directed at the liquid surface for example, or just above the surface, etc. The energy scales of gas-phase spectra were calibrated by flowing nitrous oxide calibrant into the sample compartment while the spectra were being measured. The method of calibration of the liquid spectra has been discussed [ 11. In the output of the hollow cathode lamp there is a relatively large amount of He(Ifl) impurity (its effects are evident in fig. 2, and fig. 3 has been corrected for this).
149
N
\F
filler
Me thano
I
to Pump
coolant
Ethat I‘
precision
He”2,
tube
Deconol
Fig. 1. Apparatus for maintaining and manipulating a flow of liquid inside an evacuated sample chamber. The sample is cooled and degassed in an upper compartment and then allowed to flow through a teflon tap and down a tungsten rod. Apart from the rod and tap, the apparatus is made of
glass. I
hAethanol
I
I
I
7
15
11
19
eV
Fig. 3. He(I) photoelectron spectra of methanol (-78”(Z), ethanol (-78”C), 1-heptanol (O’C) and ldecanol(2O”C). The upper spectra are those of the flowing liquid surface, the lower are those of the gas phase. The effects of He(Ip) impurity have been removed.
Fig. 2. The He(I) photoelectron spectrum of a flowing surface of liquid methanol at -78°C. The light beam grazes the surface and also produces peaks of the vapour. The small peak at low energy is caused by the presence of He(Ip) in the lamp output.
CHEMICAL PHYSICS LETTERS
Volume 127, number 2
2. Discussion The results for the vapour spectra of methanol and ethanol (fig. 3 and table 1) are in agreement with the results of other workers [2]. Vapour spectra were obtained by aiming the radiation so as just to miss the surface of the liquid. Vapour pressures (table 2) were estimated from literature data [3] and Antoine’s equation: log(pressure) = A - B/temperature
6 June 1986
Table 2 Vapour pressures (Torr) calculated by Antoine’s equation at different temperatures (“C)
methanol ethanol I-heptanol ldecanol
-78
-70
0.024 0.0048
0.065 0.015
0
20
0.012 0.024
+ C),
A, B and C being empirical constants. The spectra of methanol and ethanol (vapour and surface) were obtained at about -78°C using acetone/solid CO2 in the liquid baths, see fig. 3. Room temperature was found to be suitable for 1decanol and 0°C for 1-heptanol. Maximum flow rates of the order of 1 ml per minute were obtained, the corresponding minimum age of the surface being about 4 s. By stopping the flow and examining the surface of pendant drops, it was found that contamination of the liquid surfaces takes place, as seen by complete loss of structure of the spectrum; in methanol this takes about 2 min, in propanol about 30 s and in formamide about 5 min. Lord Rayleigh observed many years ago that a free surface of water “is almost sure to be dirty” but that a clean surface could be obtained from the “dirtiest cistern” so long as it was produced in the usual manner from a tap” [4] the reason being that flow from a tube (as in this apparatus) continuously produces fresh surface. Since
Table 1 Vertical ionisation energies for the He(I) photoelectron tra (eV). Energies of shoulders are in parentheses Methanol
Ethanol
1 -heptanol
ldecanol
vapours
10.91 12.64 15.2 (15.8) 17.5
10.70 12.12 13.19 (13.9) 15.8 17.5
(10.4) (11.0) 11.8
(10.3) 11.4
liquids
9.95 (10.97) 12.3 14.8 16.3
9.7 13.2 15.1
11.4
12.0
spec-
the photoelectron spectra of the contaminated surfaces bore little resemblance to those of the flowing surfaces it is clear that the continuous provision of fresh surface is as important with these non-aqueous liquids as it is with water. That a progressive loss of resolution in the spectra of the vapours occurs on increase of chain length in the normal, straight-chain alcohols can be seen in fig. 3. The first band (assigned as 0 lone cir) in the vapour spectra remains at 10.3 to 10.9 e in all of them but in ldecanol it has become merely a shoulder on a composite peak made up of many other bands. A spectrum attributed to methanol dimer and an ab initio calculation of ionisation energies of (CH,OH), have been reported [5] and it was suggested that the monomer peaks are so much split by interaction that a complete overlapping occurs in all the bands of the dimer, excepting only that of the lowest energy; on the assumption that the dimer represents the first stage tcwards the formation of the liquid this, if true, would invalidate the most obvious relationship between the spectra of methanol in fig. 3, namely that the four broad bands in the liquid correspond to the four bands of the vapour. By aiming the light beam so as to graze the liquid surface it was possible to obtain the spectrum of the vapour superimposed on that of the liquid and comparison of this spectrum (fig. 2) with that reported to be the spectrum of the dimer [5] leaves little doubt that only the first band (10.42 eV) can really be attributed to the dimer (explaining an otherwise anomalous feature of the purported dimer spectrum: the low intensity of the first band). However, for the lowest, Olone pair band it still seems possible to conclude that for the gas-phase dimer the ionisation en151
Volume 127, number 2
CHEMICAL PHYSICS LETTERS
ergy is intermediate between that of the gas-phase monomer (10.91 eV) and the liquid (9.95 eV).
6 June 1986
References [l ] R E. Ballard, J. Jones and E. Sutherland, Chem. Phys.
Acknowiedgement The Royal Society, the SERC and ICI are thanked for grants, C. Ellis and W. Plumbley for patiently remaking the apparatus to so many different designs.
152
Letters 112 (1984) 306. [2] K. Kimura, S. Katsumata, Y. Achiba, Y. Yamazaki and S. Iwata, Handbook of He(I) photoelectron spectra of
fundamental organic molecules (Halsted Press, New York, 1981). [ 31 R.C. Wcast, Handbook of chemistry and physics (CRC Press, Boca Raton, 1974). [4] Lord Rayleigh, Phil. Mag. 33 (1892) 363. [5] S. Tomoda and K. Kimura, Chem. Phys. 74 (1983) 121.
CHEMICAL PHYSICS LETTERS
127, number 2
6 June 1986
NEW APPARATUS FOR LIQUID SAMPLING IN ELECTRON SPECTROSCOPY. He(I) SPECTRA OF THE VAPOURS AND SURFACES OF METHANOL, ETHANOL, l-HEPTANOL AND l-DECANOL Roy BALLARD,
Jimmy JONES,
Derek READ
and Andrew
INCHLEY
School of Chemrcal Saences, Unroersrty of East Anglia, Unruerstty Plain, Norwrch, Norfolk NR4 7TJ. UK Received 13 February 1986
A new apparatus is described for maintaming either a flowing or a stationary liquid surface within an evacuated compartment for the purposes of electron spectroscopy, mass spectroscopy, etc. The liquid is made to flow down a tungsten rod which can be cooled or heated by a saturated slush bath or liquid. The position of the rod is variable so that the liquid surface can be held in the best position and orientation. By varying the rate of flow or by completely stopping the flow of liquid it is possible to examine surfaces at different times after formation and to carry out kinetic studies. The He(I) photoelectron spectra are reported of flowing surfaces of methanol and ethanol at - 78°C and also of the vapours above the flow. Spectra of I-heptanol at 0°C and 1-decanol at 20°C are also given. Contamination of these liquid surfaces with loss of spectral structure occurs in the spectrometer within 30 s to 2 min of formation.
1. Apparatus and experimental
methods
In fig. 1 is shown the apparatus for studying liquid surfaces in vacua; it is mostly made of glass. The basic principles are that (1) the sample liquid is degassed in an upper compartment, (2) the degassed liquid passes through a tap and flows down a tungsten rod to the spectrometer, (3) once past the spectrometer, the liquid is caught and frozen out in a suitably cooled tube. The upper compartment is fitted with a filler tube for the sample liquid, a tube with an anti-splash arrangement connected to a vacuum pump and a bath containing a liquid which serves to bring the sample to the correct temperature. Degassed liquid is allowed to flow into the lower half of the apparatus by means of a tap. When it flows past the tap the sample enters a glass tube passing through a second bath of liquid which either cools or heats both the liquid itself and a tungsten rod. Flowing down the rod, the sample passes close to the entrance of the spectrometer then falls into-a tubular “cold finger” in which it is retained as waste at a sufficiently low temperature to reduce its vapour pressure to a negligible value. The apparatus is attached to the sample compartment of the spectrometer by a glass tube of precise 0 009-2614/86/$0350 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
external diameter which fits into a vacuum flange by means of an O-seal. Such an arrangement has the advantage that it can be rotated so that the liquid surface can always be turned towards the spectrometer even if the flow persists in running down only one side of the rod. The flange is fitted with a variable mounting to enable adjustment of the angle of the rod with respect to the vertical; in practice this adjustment means that the distance between the liquid surface and the spectrometer entrance can be varied at will while the spectrometer is running. In addition the mounting of the He lamp is adjustable so that the radiation can be accurately directed at the liquid surface for example, or just above the surface, etc. The energy scales of gas-phase spectra were calibrated by flowing nitrous oxide calibrant into the sample compartment while the spectra were being measured. The method of calibration of the liquid spectra has been discussed [ 11. In the output of the hollow cathode lamp there is a relatively large amount of He(Ifl) impurity (its effects are evident in fig. 2, and fig. 3 has been corrected for this).
149
N
\F
filler
Me thano
I
to Pump
coolant
Ethat I‘
precision
He”2,
tube
Deconol
Fig. 1. Apparatus for maintaining and manipulating a flow of liquid inside an evacuated sample chamber. The sample is cooled and degassed in an upper compartment and then allowed to flow through a teflon tap and down a tungsten rod. Apart from the rod and tap, the apparatus is made of
glass. I
hAethanol
I
I
I
7
15
11
19
eV
Fig. 3. He(I) photoelectron spectra of methanol (-78”(Z), ethanol (-78”C), 1-heptanol (O’C) and ldecanol(2O”C). The upper spectra are those of the flowing liquid surface, the lower are those of the gas phase. The effects of He(Ip) impurity have been removed.
Fig. 2. The He(I) photoelectron spectrum of a flowing surface of liquid methanol at -78°C. The light beam grazes the surface and also produces peaks of the vapour. The small peak at low energy is caused by the presence of He(Ip) in the lamp output.
CHEMICAL PHYSICS LETTERS
Volume 127, number 2
2. Discussion The results for the vapour spectra of methanol and ethanol (fig. 3 and table 1) are in agreement with the results of other workers [2]. Vapour spectra were obtained by aiming the radiation so as just to miss the surface of the liquid. Vapour pressures (table 2) were estimated from literature data [3] and Antoine’s equation: log(pressure) = A - B/temperature
6 June 1986
Table 2 Vapour pressures (Torr) calculated by Antoine’s equation at different temperatures (“C)
methanol ethanol I-heptanol ldecanol
-78
-70
0.024 0.0048
0.065 0.015
0
20
0.012 0.024
+ C),
A, B and C being empirical constants. The spectra of methanol and ethanol (vapour and surface) were obtained at about -78°C using acetone/solid CO2 in the liquid baths, see fig. 3. Room temperature was found to be suitable for 1decanol and 0°C for 1-heptanol. Maximum flow rates of the order of 1 ml per minute were obtained, the corresponding minimum age of the surface being about 4 s. By stopping the flow and examining the surface of pendant drops, it was found that contamination of the liquid surfaces takes place, as seen by complete loss of structure of the spectrum; in methanol this takes about 2 min, in propanol about 30 s and in formamide about 5 min. Lord Rayleigh observed many years ago that a free surface of water “is almost sure to be dirty” but that a clean surface could be obtained from the “dirtiest cistern” so long as it was produced in the usual manner from a tap” [4] the reason being that flow from a tube (as in this apparatus) continuously produces fresh surface. Since
Table 1 Vertical ionisation energies for the He(I) photoelectron tra (eV). Energies of shoulders are in parentheses Methanol
Ethanol
1 -heptanol
ldecanol
vapours
10.91 12.64 15.2 (15.8) 17.5
10.70 12.12 13.19 (13.9) 15.8 17.5
(10.4) (11.0) 11.8
(10.3) 11.4
liquids
9.95 (10.97) 12.3 14.8 16.3
9.7 13.2 15.1
11.4
12.0
spec-
the photoelectron spectra of the contaminated surfaces bore little resemblance to those of the flowing surfaces it is clear that the continuous provision of fresh surface is as important with these non-aqueous liquids as it is with water. That a progressive loss of resolution in the spectra of the vapours occurs on increase of chain length in the normal, straight-chain alcohols can be seen in fig. 3. The first band (assigned as 0 lone cir) in the vapour spectra remains at 10.3 to 10.9 e in all of them but in ldecanol it has become merely a shoulder on a composite peak made up of many other bands. A spectrum attributed to methanol dimer and an ab initio calculation of ionisation energies of (CH,OH), have been reported [5] and it was suggested that the monomer peaks are so much split by interaction that a complete overlapping occurs in all the bands of the dimer, excepting only that of the lowest energy; on the assumption that the dimer represents the first stage tcwards the formation of the liquid this, if true, would invalidate the most obvious relationship between the spectra of methanol in fig. 3, namely that the four broad bands in the liquid correspond to the four bands of the vapour. By aiming the light beam so as to graze the liquid surface it was possible to obtain the spectrum of the vapour superimposed on that of the liquid and comparison of this spectrum (fig. 2) with that reported to be the spectrum of the dimer [5] leaves little doubt that only the first band (10.42 eV) can really be attributed to the dimer (explaining an otherwise anomalous feature of the purported dimer spectrum: the low intensity of the first band). However, for the lowest, Olone pair band it still seems possible to conclude that for the gas-phase dimer the ionisation en151
Volume 127, number 2
CHEMICAL PHYSICS LETTERS
ergy is intermediate between that of the gas-phase monomer (10.91 eV) and the liquid (9.95 eV).
6 June 1986
References [l ] R E. Ballard, J. Jones and E. Sutherland, Chem. Phys.
Acknowiedgement The Royal Society, the SERC and ICI are thanked for grants, C. Ellis and W. Plumbley for patiently remaking the apparatus to so many different designs.
152
Letters 112 (1984) 306. [2] K. Kimura, S. Katsumata, Y. Achiba, Y. Yamazaki and S. Iwata, Handbook of He(I) photoelectron spectra of
fundamental organic molecules (Halsted Press, New York, 1981). [ 31 R.C. Wcast, Handbook of chemistry and physics (CRC Press, Boca Raton, 1974). [4] Lord Rayleigh, Phil. Mag. 33 (1892) 363. [5] S. Tomoda and K. Kimura, Chem. Phys. 74 (1983) 121.