<PAGE>

                                                                     Exhibit 4.1

                       INTERSTATE POWER AND LIGHT COMPANY

                              OFFICER'S CERTIFICATE

                         Dated as of September 10, 2003


                             ----------------------


               Setting Forth Terms of a Series of Debt Securities

                        5.875% Senior Debentures due 2018


                             ----------------------


                            Pursuant to the Indenture
                           Dated as of August 20, 2003

<PAGE>

                              OFFICER'S CERTIFICATE

         The undersigned, the Vice President and Treasurer of Interstate Power
and Light Company, an Iowa corporation (the "Company"), hereby certifies as
provided below pursuant to Section 301 of the Indenture, dated as of August 20,
2003 (the "Indenture"), between the Company and Bank One Trust Company, National
Association (the "Trustee"). This Officer's Certificate is delivered, pursuant
to authority granted to the undersigned by resolutions adopted August 20, 2003
by the Board of Directors of the Company, for the purpose of creating and
setting forth the terms of a series of Securities to be issued pursuant to the
Indenture, and to establish the form of such Securities in accordance with
Section 201 of the Indenture. Capitalized terms not otherwise defined herein are
used as defined in the Indenture.

         1. The Board of Directors of the Company has authorized the creation by
the Company of one or more series of Securities under the Indenture through one
or more Officer's Certificates and pursuant to such authorization and in
accordance with the Indenture this Officer's Certificate is being delivered to
the Trustee to establish the terms of a series of Securities as set forth
therein.

         2. The title of the series of Securities shall be "5.875% Senior
Debentures due 2018" (herein called the "Debentures").

         3. The aggregate principal amount of the Debentures which may be
authenticated and delivered under the Indenture shall be U.S. $100,000,000,
except for Debentures authenticated and delivered upon registration of transfer
of, or in exchange for, or in lieu of, other Debentures as provided in Sections
304, 305, 306, 406 or 1206 of the Indenture. Notwithstanding the foregoing limit
on the aggregate principal amount of the Debentures, the Debentures may be
reopened in accordance with Section 301 of the Indenture.

         4. The Debentures shall be issuable in denominations of $1,000 and any
integral multiple thereof.

         5. Subject to earlier redemption, the principal of the Debentures shall
be payable in U.S. dollars on September 15, 2018.

         6. The Debentures shall bear interest at the rate of 5.875% per annum;
such interest shall accrue from September 17, 2003 (or from and including the
most recent interest payment date to which interest on the Debentures has been
paid or provided for); the Interest Payment Dates on which such interest shall
be payable shall be March 15 and September 15 in each year, commencing September
15, 2004; the Regular Record Dates for the determination of Holders to whom
interest is payable shall be (1) the Business Day next preceding each Interest
Payment Date, if the Debentures remain in book-entry only form, or (2) on the
fifteenth calendar day before each Interest Payment Date, if the Debentures do
not remain in book-entry only form. Interest on the Debentures shall be payable
in U.S. dollars.

         7. Pursuant to the Indenture, the Trustee has been appointed as the
Security Registrar for the Debentures. The Trustee is hereby further appointed
as the initial Paying Agent

<PAGE>

and transfer agent of the Debentures. The principal of and interest on the
Debentures shall be payable at the office of the Paying Agent, which shall
initially be located in the Borough of Manhattan, The City of New York.

         8. The Debentures shall be redeemable at the option of the Company at
any time in whole or from time to time in part at a Redemption Price equal to
the sum of (a) 100% of the principal amount of the Debentures being redeemed,
(b) accrued interest thereon to the Redemption Date, and (c) the Make-Whole
Amount, if any, with respect to such Debentures; provided, however, that
installments of interest on Debentures due on an Interest Payment Date which
occurs on or before any Redemption Date shall be payable to the Holders of such
Debentures who were registered Holders as of the close of business on the Record
Date immediately preceding such Interest Payment Date.

         9. The terms defined below shall, for all purposes of the Debentures
under the Indenture and this Officer's Certificate, have the meanings specified,
unless the context clearly otherwise requires or unless otherwise indicated:

         "Make-Whole Amount" means, in connection with any optional redemption,
the excess, if any, of (i) the aggregate present value as of the date of such
redemption of each dollar of principal being redeemed and the amount of
interest, exclusive of interest accrued to the Redemption Date, that would have
been payable in respect of each such dollar if such redemption had not been
made, determined by discounting, on a semi-annual basis, such principal and
interest at the Reinvestment Rate, as determined on the third Business Day
preceding the date such notice of redemption is given, from the respective dates
on which such principal and interest would have been payable if such redemption
had not been made, to the Redemption Date, over (ii) the aggregate principal
amount of the Debentures being redeemed. The Make-Whole Amount shall be
calculated by the Company and set forth in a certificate of an Authorized
Officer delivered to the Trustee, and the Trustee shall be entitled to rely on
said certificate.

         "Reinvestment Rate" means .30% plus the arithmetic mean of the yields
under the headings "Week Ending" published in the most recent Statistical
Release under the caption "Treasury Constant Maturities" for the maturity,
rounded to the nearest month, corresponding to the remaining life to maturity,
as of the payment date of the principal amount of the Debentures being redeemed.
If no maturity exactly corresponds to such maturity, yields for the two
published maturities most closely corresponding to such maturity shall be
calculated pursuant to the immediately preceding sentence and the Reinvestment
Rate shall be interpolated or extrapolated from such yields on a straight-line
basis, rounding in each of such relevant periods to the nearest month. For
purposes of calculating the Reinvestment Rate, the most recent Statistical
Release published prior to the date of determination of the Make-Whole Amount
shall be used. If the format or content of the Statistical Release changes in a
manner that precludes determination of the Treasury yield in the above manner,
then the Treasury yield shall be determined in the manner that most closely
approximates the above manner, as reasonably determined by the Company.

         "Statistical Release" means the statistical release designated
"H.15(519)" or any successor publication which is published weekly by the
Federal Reserve System and which

                                       -2-

<PAGE>

reports yields on actively traded United States government securities adjusted
to constant maturities, or, if such statistical release is not published at the
time of any required determination under the Indenture, then such other
reasonably comparable index which shall be designated by the Company.

         10. The Debentures shall not be subject to any sinking fund and shall
not be repurchasable or redeemable at the option of a Holder.

         11. The Debentures shall not be convertible into other securities of
the Company or exchangeable for securities of another issuer.

         12. Satisfaction and discharge under Section 701 of the Indenture shall
be applicable to the Debentures; provided, however, that prior to any such
satisfaction and discharge, the Company shall have delivered to the Trustee an
Opinion of Counsel stating that (a) the Company has received from the Internal
Revenue Service a letter ruling, or there has been published by the Internal
Revenue Service a revenue ruling, or (b) since the date of execution of this
Officer's Certificate, there has been a change in the applicable Federal income
tax law, in either case to the effect that, and based thereon such opinion shall
confirm that, the Holders of such Outstanding Securities will not recognize
income, gain or loss for Federal income tax purposes as a result of such
satisfaction and discharge and will be subject to Federal income tax on the same
amounts, in the same manner and at the same times as would have been the case if
such satisfaction and discharge had not occurred.

         13. The Debentures shall initially be issued in whole in the form of
one or more permanent global Securities. The Depository Trust Company ("DTC"), a
clearing agency registered under the Securities Exchange Act of 1934, as
amended, shall initially serve as the depositary for such global Security or
Securities. For so long as DTC shall be the depositary, all Debentures shall be
registered in its name or in the name of a nominee thereof. While the Debentures
are evidenced by one or more global Securities, the depositary or its nominee,
as the case may be, shall be the sole Holder thereof for all purposes under the
Indenture. Neither the Company nor the Trustee shall have any responsibility or
obligation to the depositary's participants or the beneficial owners for whom
they act with respect to their receipt from the depositary of payments on the
Debentures or notices given under the Indenture. The global Security or
Securities provided for hereunder shall bear such legend or legends as may be
required from time to time by the depositary.

         14. Except as herein described, Debentures in definitive form will not
be issued. Notwithstanding the foregoing, in the event the Company decides to
discontinue the use of global Securities, any Event of Default has occurred and
is continuing or DTC is at any time unwilling, unable or ineligible to continue
as depositary and a successor depositary is not appointed by the Company within
90 days, the Company shall issue individual Debentures in certificated form to
owners of "book-entry" ownership interests in exchange for the Debentures held
by DTC or its nominee, as the case may be. In such instance, an owner of a
"book-entry" ownership interest will be entitled to physical delivery of
certificates equal in principal amount to such "book-entry" ownership interest
and to have such certificates registered in its name.

                                      -3-

<PAGE>

Individual certificates so issued will be issued in denominations of $1,000 or
any multiple thereof.

         15. Additional terms regarding the Debentures are as set forth in the
form of the Debentures set forth below.

         16. The form of the Debentures shall be substantially as follows:

                                      -4-

<PAGE>

     Unless this certificate is presented by an authorized representative of The
Depository Trust Company, a New York corporation ("DTC"), to the Company (as
defined below) or its agent for registration of transfer, exchange or payment,
and any certificate issued is registered in the name of Cede & Co. or in such
other name as is requested by an authorized representative of DTC (and any
payment is made to Cede & Co. or to such other entity as is requested by an
authorized representative of DTC), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR
VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL inasmuch as the registered
owner hereof, Cede & Co., has an interest herein.

                       INTERSTATE POWER AND LIGHT COMPANY

                        5.875% SENIOR DEBENTURES DUE 2018

No.                                                                $____________
                                                           CUSIP No. 461070 AA 2

     INTERSTATE POWER AND LIGHT COMPANY, a corporation duly organized and
existing under the laws of the State of Iowa (herein called the "Company," which
term includes any successor Person under the Indenture hereinafter referred to),
for value received, hereby promises to pay to Cede & Co., or registered assigns,
the principal sum of ___________ ________ DOLLARS ($___________) on September
15, 2018 and to pay interest on said principal sum from September 17, 2003, or
from and including the most recent interest payment date to which interest has
been paid or duly provided for, semi-annually, in arrears, on March 15 and
September 15 of each year (each such date, an "Interest Payment Date"),
commencing March 15, 2004 at the rate of 5.875% per annum to, but not including,
the date on which the principal hereof is paid or made available for payment.
The amount of interest payable for any period will be computed on the basis of
twelve 30-day months and a 360-day year. The interest so payable, and punctually
paid or duly provided for, on any Interest Payment Date will, as provided in the
Indenture, be paid to the Person in whose name this Debenture (or one or more
Predecessor Securities, as defined in the Indenture) is registered at the close
of business, on the Regular Record Date for such interest, which shall be (1)
the Business Day next preceding each such Interest Payment Date, if the
Debentures remain in book-entry only form, or (2) on the fifteenth calendar day
before each interest payment date, if the Debentures do not remain in book-entry
only form. Any such interest installment not so punctually paid or duly provided
for shall forthwith cease to be payable to the registered Holder on such Regular
Record Date and may either be paid to the Person in whose name this Debenture
(or one or more Predecessor Securities) is registered at the close of business
on a Special Record Date for the payment of such Defaulted Interest to be fixed
by the Trustee, notice whereof shall be given to Holders of Debentures not later
than 10 days prior to such Special Record Date, or be paid at any time in any
other lawful manner not inconsistent with the requirements of any securities
exchange on which the Debenture may be listed, or any book-entry system which
may be applicable to this Debenture and upon such notice as may be required by
such exchange or system, all as more fully provided in the Indenture.

     Payment of the principal of and interest on any Debenture that is not a
global Debenture will be made at the office or agency of the Company maintained
for that purpose in The City of New York; provided, however, that at the option
of the Company payment of interest may be made (i) by check mailed to the
address of the Person entitled thereto as such address shall appear in the
Security Register or (ii) by wire transfer in immediately available funds at
such place and to such account as may be designated by the Person entitled
thereto as specified in the Security Register. Payment of principal of and
interest on any global Debenture will be made to DTC or its nominee, as the case
may be, as the sole registered owner and the sole Holder of the global Debenture
for all purposes under the Indenture. Payment of the principal of and interest
on this Debenture will be made in such coin or currency of the United States of
America as at the time of payment is legal tender for payment of public and
private debts.

     Additional provisions of this Debenture are continued on the two pages
following the execution and authentication of this Debenture and such provisions
have the same effect as though fully set forth in this place.

     Unless the certificate of authentication hereon has been executed by or on
behalf of the Trustee by manual signature, this Debenture shall not be entitled
to any benefit under the Indenture or be valid or obligatory for any purpose.

                                       -5-

<PAGE>

     IN WITNESS WHEREOF, INTERSTATE POWER AND LIGHT COMPANY has caused this
instrument to be duly executed under its corporate seal.


Dated: __________ __, 20__


                                              INTERSTATE POWER AND LIGHT COMPANY



                                              By:_______________________________
                                                 Name
                                                 Title

Attest:



_________________________________
Authorized Officer

                     Trustee's Certificate of Authentication

     This is one of the Debentures of the series designated herein referred to
in the within-mentioned Indenture.

                                   BANK ONE TRUST COMPANY, NATIONAL ASSOCIATION,
                                         as Trustee



                                   By:_______________________________
                                         Authorized Officer

                                       -6-

<PAGE>

                       INTERSTATE POWER AND LIGHT COMPANY
                        5.875% SENIOR DEBENTURES DUE 2018

     This Debenture is one of a duly authorized issue of Debentures of the
Company, designated as its "5.875% Senior Debentures due 2018" (herein called
the "Debentures"), in aggregate principal amount of $100,000,000, issued under
an Indenture, dated as of August 20, 2003 (herein call the "Indenture"), between
the Company and Bank One Trust Company, National Association (the "Trustee"), to
which Indenture and the Officer's Certificate, dated September 10, 2003, setting
forth the terms and conditions of the Debentures, reference is hereby made for a
statement of the respective rights, limitations of rights, duties and immunities
thereunder of the Trustee, the Company and the Holders of the Debentures, and of
the terms upon which the Debenture are, and are to be, authenticated and
delivered.

     The Company may redeem the Debentures at any time at the Company's option,
in whole or in part, at a redemption price equal to the sum of the principal
amount of the Debentures the Company redeems, accrued interest on that principal
amount to the redemption date and the Make-Whole Amount (as defined in the
Officer's Certificate), if any, with respect to those Debentures.

     If an Event of Default with respect to the Debentures shall occur and be
continuing, the principal of the Debentures may be declared due and payable in
the manner, with the effect and subject to the conditions, provided in the
Indenture.

     The Indenture contains provisions for satisfaction and discharge at any
time of the entire indebtedness of this Debenture upon compliance by the Company
with certain conditions set forth in the Indenture.

     The Indenture contains provisions permitting the Company and the Trustee,
with the consent of the Holders of not less than a majority in principal amount
of the Outstanding Debentures, to modify the Indenture in a manner affecting the
rights of the Holders of the Debentures; provided that no such modification may,
without the consent of the Holder of each Outstanding Debenture, (i) change the
Stated Maturity of, the principal of, or any installment of principal of or
interest on (except as provided in Section 312 of the Indenture), any Debenture,
or reduce the principal amount thereof or the rate of interest thereon (or the
amount of any installment of interest thereon) or change the method of
calculating such rate or reduce any premium payable upon the redemption thereof,
or change the coin or currency (or other property), in which the Debentures or
any premium or the interest thereon is payable, or impair the right to institute
suit for the enforcement of any such payment on or after the Stated Maturity of
any Debenture or (ii) reduce the percentage in principal amount of the
Outstanding Debentures of any series or any Tranche thereof, the consent of the
Holders of which is required for any such modification of the Indenture. The
Indenture also contains provisions permitting Holders of specified percentages
in principal amount of the Debentures at the time Outstanding, on behalf of the
Holders of all Debentures, to waive compliance by the Company with certain
provisions of the Indenture and certain past defaults under the Indenture and
their consequences. Any such consent or waiver by the Holder of this Debenture
shall be conclusive and binding upon such Holder and upon all future Holders of
this Debenture and of any Debenture issued upon the registration of transfer
hereof or in exchange herefor or in lieu hereof, whether or not notation of such
consent or waiver is made upon this Debenture.

     No reference herein to the Indenture and no provision of this Debenture or
of the Indenture shall alter or impair the obligation of the Company, which is
absolute and unconditional, to pay the principal of and interest on this
Debenture at the times, place and rate, and in the coin or currency, herein
prescribed.

     As provided in the Indenture and subject to certain limitations, including,
if this Debenture is a global Debenture, the limitations set forth on the first
page hereof, therein set forth, the transfer of this Debenture is registrable in
the Security Register, upon surrender of this Debenture for registration of
transfer at the office or agency of the Company in The City of New York
maintained for such purpose, duly endorsed by, or accompanied by a written
instrument of transfer in form satisfactory to the Company and the Security
Registrar duly executed by, the Holder hereof or his attorney duly authorized in
writing, and thereupon one or more new Debentures, of authorized denominations
and for the same aggregate principal amount, will be issued to the designated
transferee or transferees. No service charge shall be made for any such
registration of transfer or exchange, but the Company may require payment of a
sum sufficient to cover any tax or other governmental charge payable in
connection therewith.

     Prior to due presentment of this Debenture for registration of transfer,
the Company, the Trustee and any agent of the Trustee may treat the Person in
whose name this Debenture is registered as the owner hereof for all purposes,
whether or not this Debenture be overdue, and neither the Company, the Trustee
nor any such agent shall be affected by notice to the contrary.

     The Debentures are issuable only in registered form without coupons in
denominations of $1,000 and any integral multiple thereof.

                                       -7-

<PAGE>

     All terms used in this Debenture which are defined in the Indenture shall
have the meanings assigned to them in the Indenture.

     THIS DEBENTURE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
LAWS OF THE STATE OF NEW YORK WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES
THEREOF.

                                       -8-

<PAGE>

          IN WITNESS WHEREOF, the undersigned has set his hand as of the day and
year first above written.

                                     INTERSTATE POWER AND LIGHT COMPANY



                                         By:   /s/ Thomas L. Hanson
                                               -------------------------
                                               Thomas L. Hanson
                                               Vice President and Treasurer

                                       -9-