276
P. Madhusudhan et al.
T D =
√
2m
2U
D
(20)
The + and − signs in T s are for the ions having forward and backward initial
velocities, respectively.
If the initial energy (U 0 ) of the ion is zero, then the total TOF can be written as
T =
√
m
2U
2
√
k.s +
2
√
k
√
k + 1
d + D
(21)
To minimise the spread in the TOFs of ions, the space focusing condition should be
achieved. This is based on the principle that ions formed closer to the detector gain less
energy than the ions formed further away from the detector. This causes the ions with
larger energy to overtake those with lesser energy. Let s 0 be the centre of the ionization
region and the total spread in position be Δs. Then the maximum TOF difference
is between ions having initial positions s max = s 0 +
1
2
Δs, and s min = s 0 −
1
2
Δs. We
wish to find that position in space where these ions (with zero initial energy) overtake
and minimise the TOF spread between the ions formed at s max and s min . For that, we
set
∂T
∂s
0,s 0
= 0. This gives us
D = 2s 0 k
3/2
0
1 −
1
k 0 + k
1/2
0
.
d
s 0
,
(22)
where k 0 = (s 0 E s + dE d )/s 0 E s . Since s 0 , D, and d are fixed quantities so the space
focusing condition can be achieved by the optimization of E s /E d for the double-field
TOF MS. For the single-field TOF MS, d = 0, E d = 0, and k 0 = 1. Thus, the space
focusing condition for the single-field TOF MS is given by D = 2s 0 .
By analyzing the TOF spectrum, we can get the relative yield of species with
respect to the parent ion. In the next section, the coincidence technique is discussed, which can give information about various dissociative channels of fragmenting molecular ions.
5.2 Photoelectron-Photoion Coincidence Technique
(PEPICO)
The time-of-flight spectrometer is useful for ion yield analysis. The ions in the TOF
spectrum are originated from various dissociative channels of precursor ions. The
time of flight spectrometer is not capable of giving information on dissociative channels. For observing various dissociative channels of precursor ions, a new ion detection method known as coincidence technique was developed, which can be considered
as an add-on to the TOF spectrometer. In this technique, a single collision condition
P. Madhusudhan et al.
T D =
√
2m
2U
D
(20)
The + and − signs in T s are for the ions having forward and backward initial
velocities, respectively.
If the initial energy (U 0 ) of the ion is zero, then the total TOF can be written as
T =
√
m
2U
2
√
k.s +
2
√
k
√
k + 1
d + D
(21)
To minimise the spread in the TOFs of ions, the space focusing condition should be
achieved. This is based on the principle that ions formed closer to the detector gain less
energy than the ions formed further away from the detector. This causes the ions with
larger energy to overtake those with lesser energy. Let s 0 be the centre of the ionization
region and the total spread in position be Δs. Then the maximum TOF difference
is between ions having initial positions s max = s 0 +
1
2
Δs, and s min = s 0 −
1
2
Δs. We
wish to find that position in space where these ions (with zero initial energy) overtake
and minimise the TOF spread between the ions formed at s max and s min . For that, we
set
∂T
∂s
0,s 0
= 0. This gives us
D = 2s 0 k
3/2
0
1 −
1
k 0 + k
1/2
0
.
d
s 0
,
(22)
where k 0 = (s 0 E s + dE d )/s 0 E s . Since s 0 , D, and d are fixed quantities so the space
focusing condition can be achieved by the optimization of E s /E d for the double-field
TOF MS. For the single-field TOF MS, d = 0, E d = 0, and k 0 = 1. Thus, the space
focusing condition for the single-field TOF MS is given by D = 2s 0 .
By analyzing the TOF spectrum, we can get the relative yield of species with
respect to the parent ion. In the next section, the coincidence technique is discussed, which can give information about various dissociative channels of fragmenting molecular ions.
5.2 Photoelectron-Photoion Coincidence Technique
(PEPICO)
The time-of-flight spectrometer is useful for ion yield analysis. The ions in the TOF
spectrum are originated from various dissociative channels of precursor ions. The
time of flight spectrometer is not capable of giving information on dissociative channels. For observing various dissociative channels of precursor ions, a new ion detection method known as coincidence technique was developed, which can be considered
as an add-on to the TOF spectrometer. In this technique, a single collision condition