@article{PismaZhETF.9.216,
    title = {Three-level Gas Laser},
    author = {Beterov, I. M. and Chebotaev, P.},
    journal = {πΙΣΨΝΑ Χ φότζ},
    volume = {9},
    issue = {4},
    pages = {216},
    year = {1969},
    doi = {},
    url = {http://jetpletters.ru/ps/0/article_25312.shtml},
}