The director may, with the approval of the commissioner, under rules adopted under RSA 541-A, establish a per diem rate of payment in lieu of reimbursement for expenses incurred in the discharge of their duties for all employees of the state police who are regularly assigned to field work in either a patrol or investigatory capacity. Employees regularly assigned either to troop headquarters or to the central administrative offices shall not be entitled to such per diem payment. The director may authorize the payment of said expenses monthly, from funds appropriated to the division of state police, provided, however, said payment shall include reimbursement only for those days actually worked in each month. The director, prior to authorizing any such payments, shall, with the approval of the commissioner, submit to the commissioner of administrative services lists of all employees for whom such payments are authorized by him, which shall include, for each such employee, his classification and duty assignment, and the specific type and kind of expenses for which he is being reimbursed. He shall, with the approval of the commissioner, submit to the commissioner of administrative services, at like time and in like manner and detail, any deletions or additions to said lists.