A method for measuring the content of a gas compound from a gas mixture, wherein the gas compound consists of gas compound molecules, and the gas compound molecule is not an alkalihalide molecule, wherein the alkalihalide molecule has form MH a, wherein M is an alkali metal atom from the group of Li, Na, K, Ru, Cs, and Fr, and H a is a halogen atom from the group of F, Cl, Br, I, and At, the method comprising - generating a light beam, wherein the light beam comprises photons having a beam wavelength (λ b), - guiding the light beam to a first optical path, wherein - the first optical path runs through a space containing the gas mixture comprising the gas compound, whereby the light beam is attenuated to an attenuated light beam, - detecting a first value indicative of a first intensity (I k0) of the attenuated light beam, - generating a last light pulse (710, 710a), wherein the last light pulse (710, 710a) comprises photons (515, 515b) having a first pulse wavelength (λ p1), and - optionally generating another light pulse (710b, 710c), characterized in that - the light beam is monochromatic or essentially monochromatic having the beam wavelength (λ b) - the last light pulse (710, 710a) is monochromatic or essentially monochromatic having the first pulse wavelength (λ p1), the method further comprising - guiding at least part of the last light pulse (710, 710a) to a second optical path, which runs through the space, such that the second optical path is essentially parallel to the first optical path, - dissociating at least part of the gas compound molecules or at least part of excited gas compound molecules on the first optical path to at least two dissociated parts (520, 525, 530, 532, 534) using the last light pulse (710, 710a), or the another light pulse (710b, 710c), - dissociating (i) at least part of the gas compound molecules, (ii) at least part of excited gas compound molecules, (iii) at least part of the dissociated parts (530, 534), or (iv) at least part of the excited dissociated parts (530, 534) on the first optical path to first part atoms, molecules, ions, or radicals, and to another part (525, 532, 536) using the last light pulse (710, 710a), whereby the light beam is further attenuated to an attenuated light beam by absorption to the first part atoms, molecules, ions, or radicals on the first optical path, - selecting the beam wavelength (λ b) such that it corresponds to the absorption profile of the first part, - detecting a second value indicative of a second intensity (I k) of the attenuated light beam, and - determining, using the first value and the second value, the gas compound content of the gas mixture. - generating a light beam, wherein the light beam comprises photons having a beam wavelength (λ b), - guiding the light beam to a first optical path, wherein - the first optical path runs through a space containing the gas mixture comprising the gas compound, whereby the light beam is attenuated to an attenuated light beam, - detecting a first value indicative of a first intensity (I k0) of the attenuated light beam, - generating a last light pulse (710, 710a), wherein the last light pulse (710, 710a) comprises photons (515, 515b) having a first pulse wavelength (λ p1), and - optionally generating another light pulse (710b, 710c), - the light beam is monochromatic or essentially monochromatic having the beam wavelength (λ b) - the last light pulse (710, 710a) is monochromatic or essentially monochromatic having the first pulse wavelength (λ p1), the method further comprising - guiding at least part of the last light pulse (710, 710a) to a second optical path, which runs through the space, such that the second optical path is essentially parallel to the first optical path, - dissociating at least part of the gas compound molecules or at least part of excited gas compound molecules on the first optical path to at least two dissociated parts (520, 525, 530, 532, 534) using the last light pulse (710, 710a), or the another light pulse (710b, 710c), - dissociating (i) at least part of the gas compound molecules, (ii) at least part of excited gas compound molecules, (iii) at least part of the dissociated parts (530, 534), or (iv) at least part of the excited dissociated parts (530, 534) on the first optical path to first part atoms, molecules, ions, or radicals, and to another part (525, 532, 536) using the last light pulse (710, 710a), whereby the light beam is further attenuated to an attenuated light beam by absorption to the first part atoms, molecules, ions, or radicals on the first optical path, - selecting the beam wavelength (λ b) such that it corresponds to the absorption profile of the first part, - detecting a second value indicative of a second intensity (I k) of the attenuated light beam, and - determining, using the first value and the second value, the gas compound content of the gas mixture.